Add description of concept documents

[Docs] Add description of concept documents

Co-authored-by: Jeremy Walker <[email protected]>

reference/implementing-a-concept-exercise.md

@@ -5,14 +5,17 @@ To implement an Emacs Lisp concept exercise, the following files must be added:
 ```
 languages
 └── emacs-lisp
+    ├── concepts
+    |   └── <CONCEPT_SLUG>
+    |       ├── about.md
+    |       └── links.json
     └── exercises
         └── concept
             └── <SLUG>
                 ├── .docs
                 |   ├── instructions.md
                 |   ├── introduction.md
-                |   ├── hints.md
-                |   └── after.md
+                |   └── hints.md
                 ├── .meta
                 |   |── design.md
                 |   └── Example.el