Refactor headline -> heading for consistency

Motivation for this change:
- https://lists.gnu.org/r/emacs-orgmode/2021-07/msg00563.html
- https://lists.gnu.org/r/emacs-orgmode/2021-08/msg00056.html

While that change hasn't yet entered the Org mode codebase of today
<2022/01/03>, I like the idea of using the term "heading" instead of
"headline".

Definitions from WordNet (ref:
https://lists.gnu.org/r/emacs-orgmode/2021-07/msg00576.html):

  heading
      n 1: a line of text serving to indicate what the passage below
           it is about; "the heading seemed to have little to do with
           the text" [syn: {heading}, {header}, {head}]
  headline
      n 1: the heading or caption of a newspaper article [syn:
           {headline}, {newspaper headline}]

README.org

@@ -388,15 +388,15 @@ to an empty string (=""=).
   #[[https://github.com/kaushalmodi/ox-hugo/issues/89][89]].
 - File-based exports must now use =#+hugo_tags= to set the post tags.
 - Subtree-based exports can use the =EXPORT_HUGO_TAGS= property to
-  override Org-style tags on the same headline (and the ones inherited
+  override Org-style tags on the same heading (and the ones inherited
   from Org-style tags from any of the parent subtrees and
   =#+filetags=).
   - Note that for subtree-based exports, =#+filetags= can be used to
     set tags globally in the file. Earlier =#+tags= was used for that
     purpose.
 - Subtree-based exports can use the =EXPORT_HUGO_CATEGORIES= property
   to override Org-style categories (tags with "@" prefix) on the same
-  headline (and the ones inherited from Org-style categories from any
+  heading (and the ones inherited from Org-style categories from any
   of the parent subtrees and =#+filetags=).
   - Note that for subtree-based exports, =#+filetags= can be used to
     set categories (tags with "@") globally in the file.
@@ -409,7 +409,7 @@ See the new section added to documentation:
   parameter in the =figure= tag in the HTML generated by Hugo.
 ** v0.3.2 <2017-10-24 Tue>
 *** Fixes
-- Fix issue with headline metadata parsing (ALLTAGS, CLOSED, TODO)
+- Fix issue with heading metadata parsing (ALLTAGS, CLOSED, TODO)
   when a post Org heading was immediately followed by that post's
   sub-heading.  This issue was seen in subtree-based exports
   #[[https://github.com/kaushalmodi/ox-hugo/issues/87][87]].

doc/ox-hugo-manual.org

@@ -218,15 +218,15 @@
   {{{issue(89)}}}.
 - File-based exports must now use =#+hugo_tags= to set the post tags.
 - Subtree-based exports can use the =EXPORT_HUGO_TAGS= property to
-  override Org-style tags on the same headline (and the ones inherited
+  override Org-style tags on the same heading (and the ones inherited
   from Org-style tags from any of the parent subtrees and
   =#+filetags=).
   - Note that for subtree-based exports, =#+filetags= can be used to
     set tags globally in the file. Earlier =#+tags= was used for that
     purpose.
 - Subtree-based exports can use the =EXPORT_HUGO_CATEGORIES= property
   to override Org-style categories (tags with "@" prefix) on the same
-  headline (and the ones inherited from Org-style categories from any
+  heading (and the ones inherited from Org-style categories from any
   of the parent subtrees and =#+filetags=).
   - Note that for subtree-based exports, =#+filetags= can be used to
     set categories (tags with "@") globally in the file.
@@ -239,7 +239,7 @@ See the new section added to documentation:
   parameter in the =figure= tag in the HTML generated by Hugo.
 ** v0.3.2 <2017-10-24 Tue>
 *** Fixes
-- Fix issue with headline metadata parsing (ALLTAGS, CLOSED, TODO)
+- Fix issue with heading metadata parsing (ALLTAGS, CLOSED, TODO)
   when a post Org heading was immediately followed by that post's
   sub-heading.  This issue was seen in subtree-based exports
   {{{issue(87)}}}.
@@ -1702,12 +1702,12 @@ Though, the advantage of Org-generated TOC is that you get finer
 control on:
 - *Where* to include the TOC --- Location of the =#+toc= keyword in the
   Org content.
-- How many headlines to include in the TOC --- /Example: =#+toc:
-  headlines 2=/ or =:EXPORT_OPTIONS: toc:2=.
-- Whether you want *all* the headlines in the TOC to be numbered or
+- How many headings to include in the TOC --- /Example: =#+toc:
+  headings 2=/ or =:EXPORT_OPTIONS: toc:2=.
+- Whether you want *all* the headings in the TOC to be numbered or
   not --- See =org-hugo-export-with-section-numbers=.
-- Whether you want *only some* headlines numbered (both in post body
-  and the TOC) --- Set the =UNNUMBERED= property of that headline to
+- Whether you want *only some* headings numbered (both in post body
+  and the TOC) --- Set the =UNNUMBERED= property of that heading to
   =t=.
 - Whether you want to list the sub-headings *only from the current
   heading in the post* --- /Example: ~#+toc: headlines 1 local~/.
@@ -2246,7 +2246,7 @@ Hit =C-x C-s= and be in awe as the browser auto-refreshes to the
 If you do not want to manually type the =EXPORT_FILE_NAME= for each
 new post, here is an example Org Capture template can help:
 #+begin_src emacs-lisp
-;; Populates only the EXPORT_FILE_NAME property in the inserted headline.
+;; Populates only the EXPORT_FILE_NAME property in the inserted heading.
 (with-eval-after-load 'org-capture
   (defun org-hugo-new-subtree-post-capture-template ()
     "Returns `org-capture' template string for new Hugo post.

ox-blackfriday.el

@@ -130,15 +130,15 @@ tag needs to be `python'."
 ;;; Miscellaneous Helper Functions
 
 ;;;; Table of contents
-(defun org-blackfriday-format-toc (headline info)
-  "Return an appropriate table of contents entry for HEADLINE.
+(defun org-blackfriday-format-toc (heading info)
+  "Return an appropriate table of contents entry for HEADING.
 
 INFO is a plist used as a communication channel."
-  (let* ((title (org-export-data (org-export-get-alt-title headline info) info))
-         (level (1- (org-element-property :level headline)))
+  (let* ((title (org-export-data (org-export-get-alt-title heading info) info))
+         (level (1- (org-element-property :level heading)))
          (indent (concat (make-string (* level 2) ? )))
-         (anchor (or (org-element-property :CUSTOM_ID headline)
-                     (org-export-get-reference headline info))))
+         (anchor (or (org-element-property :CUSTOM_ID heading)
+                     (org-export-get-reference heading info))))
     (concat indent "- [" title "]" "(#" anchor ")")))
 
 ;;;; Footnote section
@@ -651,14 +651,14 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 CONTENTS is the transcoded contents string.  INFO is a plist
 holding export options."
   (let* ((depth (plist-get info :with-toc))
-         (headlines (and depth (org-export-collect-headlines info depth)))
-         (toc-tail (if headlines "\n\n" ""))
+         (headings (and depth (org-export-collect-headlines info depth)))
+         (toc-tail (if headings "\n\n" ""))
          (toc-string ""))
 
-    (when headlines
-      (dolist (headline headlines)
+    (when headings
+      (dolist (heading headings)
         (setq toc-string (concat toc-string
-                                 (org-blackfriday-format-toc headline info)
+                                 (org-blackfriday-format-toc heading info)
                                  "\n"))))
     (org-trim (concat toc-string toc-tail contents "\n"
                       (org-blackfriday-footnote-section nil info)))))
@@ -1242,7 +1242,7 @@ asynchronously.  The resulting buffer should be accessible
 through the `org-export-stack' interface.
 
 When optional argument SUBTREEP is non-nil, export the sub-tree
-at point, extracting information from the headline properties
+at point, extracting information from the heading properties
 first.
 
 When optional argument VISIBLE-ONLY is non-nil, don't export
@@ -1280,7 +1280,7 @@ asynchronously.  The resulting file should be accessible through
 the `org-export-stack' interface.
 
 When optional argument SUBTREEP is non-nil, export the sub-tree
-at point, extracting information from the headline properties
+at point, extracting information from the heading properties
 first.
 
 When optional argument VISIBLE-ONLY is non-nil, don't export