Trac #33601: Documentation: Add 'furo' theme

As discussed in https://trac.sagemath.org/ticket/33309#comment:28, we
add Furo (https://pradyunsg.me/furo/) as a possible theme for the
documentation.
After gaining some experience with it, the goal would be to make it the
default theme for sage's docs.

To test, run
{{{
make furo
make doc-clean; make doc-uninstall; make
}}}

To go back to the classic sage theme, uninstall furo using `./sage -pip
uninstall furo`.

Known issues (to be fixed as follow-ups/upstream):
- Reference manual index page and many sub-pagaes don't have a table of
contents on the left https://516dfe85b21eafae4f042320e5dcf61ad14fe780
--sagemath-tobias.netlify.app/reference/
- Index page is missing/not working
https://516dfe85b21eafae4f042320e5dcf61ad14fe780--sagemath-
tobias.netlify.app/
- sage logo in the side bar should link to the top of the Sage manuals

URL: https://trac.sagemath.org/33601
Reported by: gh-tobiasdiez
Ticket author(s): Tobias Diez
Reviewer(s): Kwankyu Lee

.github/workflows/doc-build.yml

@@ -24,6 +24,7 @@ jobs:
           # Reuse built SAGE_LOCAL contained in the Docker image 
           ./bootstrap
           ./configure --enable-build-as-root --prefix=/sage/local --with-sage-venv --enable-download-from-upstream-url
+          make furo
 
       - name: Build
         run: make doc-html

.vscode/settings.json

@@ -22,6 +22,7 @@
     ],
     "python.testing.unittestEnabled": false,
     "cSpell.words": [
+        "furo"
         "Conda",
         "Cython"
     ],

build/pkgs/furo/SPKG.rst

@@ -0,0 +1,16 @@
+furo: A clean customizable Sphinx documentation theme
+=====================================================
+
+Description
+-----------
+
+A clean customizable Sphinx documentation theme.
+
+License
+-------
+
+Upstream Contact
+----------------
+
+https://pypi.org/project/furo/
+

build/pkgs/furo/dependencies

@@ -0,0 +1,9 @@
+$(PYTHON) beautifulsoup4 sphinx pygments | $(PYTHON_TOOLCHAIN)
+
+----------
+All lines of this file are ignored except the first.
+
+From https://github.com/pradyunsg/furo/blob/b4c8010cff5e494a93d617069897964c60b820e9/pyproject.toml#L18-L22
+  "beautifulsoup4",
+  "sphinx ~= 4.0",
+  "pygments ~= 2.7",

build/pkgs/furo/requirements.txt

@@ -0,0 +1 @@
+furo

build/pkgs/furo/type

@@ -0,0 +1 @@
+optional

build/pkgs/mathjax/SPKG.rst

@@ -13,7 +13,6 @@ License
 
 Apache License, version 2.0
 
-
 Upstream Contact
 ----------------
 
@@ -22,16 +21,8 @@ Home page: https://www.mathjax.org/
 Dependencies
 ------------
 
-None.
-
-
 Special Update/Build Instructions
 ---------------------------------
 
-None.
-
-
 Patches
 -------
-
-None.