Refactor wiki Markdown documentation

doc/markdown/wiki.md

-# Wiki-Specific Markdown
+# Wiki-specific Markdown
 
-## Table of Contents
+This page has information related to wiki-specific Markdown. For more
+information on GitLab's Markdown, see the [main Markdown document](./markdown.md).
 
-* [Links to Other Wiki Pages](#links-to-other-wiki-pages)
-  * [Direct Page Link](#direct-page-link)
-  * [Direct File Link](#direct-file-link)
-  * [Hierarchical Link](#hierarchical-link)
-  * [Root Link](#root-link)
+## Table of contents
 
-## Links to Other Wiki Pages
+* [Links to other wiki pages](#links-to-other-wiki-pages)
+  * [Direct page link](#direct-page-link)
+  * [Direct file link](#direct-file-link)
+  * [Hierarchical link](#hierarchical-link)
+  * [Root link](#root-link)
+
+## Links to other wiki pages
 
 You can link to other pages on your wiki in a few different ways.
 
-### Direct Page Link
+### Direct page link
 
-A link which just includes the slug for a page will point to that page, _at the base level of the wiki_.
+A link which just includes the slug for a page will point to that page,
+_at the base level of the wiki_.
 
-1. This snippet would link to a `documentation` page at the root of your wiki.
+This snippet would link to a `documentation` page at the root of your wiki:
 
 ```markdown
 [Link to Documentation](documentation)
 ```
 
-### Direct File Link
+### Direct file link
 
 Links with a file extension point to that file, _relative to the current page_.
 
-1. If this snippet was placed on a page at `<your_wiki>/documentation/related`, it would link to `<your_wiki>/documentation/file.md`.
+If this snippet was placed on a page at `<your_wiki>/documentation/related`,
+it would link to `<your_wiki>/documentation/file.md`:
 
-	```markdown
-	[Link to File](file.md)
-	```
+```markdown
+[Link to File](file.md)
+```
 
-### Hierarchical Link
+### Hierarchical link
 
-A link can be constructed relative to the current wiki page using `./<page>`, `../<page>`, etc.
+A link can be constructed relative to the current wiki page using `./<page>`,
+`../<page>`, etc.
 
-1. If this snippet was placed on a page at `<your_wiki>/documentation/main`, it would link to `<your_wiki>/documentation/related`.
+- If this snippet was placed on a page at `<your_wiki>/documentation/main`,
+  it would link to `<your_wiki>/documentation/related`:
 
 	```markdown
 	[Link to Related Page](./related)
 	```
 
-1. If this snippet was placed on a page at `<your_wiki>/documentation/related/content`, it would link to `<your_wiki>/documentation/main`.
+- If this snippet was placed on a page at `<your_wiki>/documentation/related/content`,
+  it would link to `<your_wiki>/documentation/main`:
 
 	```markdown
 	[Link to Related Page](../main)
 	```
 
-1. If this snippet was placed on a page at `<your_wiki>/documentation/main`, it would link to `<your_wiki>/documentation/related.md`.
+- If this snippet was placed on a page at `<your_wiki>/documentation/main`,
+  it would link to `<your_wiki>/documentation/related.md`:
 
 	```markdown
 	[Link to Related Page](./related.md)
 	```
 
-1. If this snippet was placed on a page at `<your_wiki>/documentation/related/content`, it would link to `<your_wiki>/documentation/main.md`.
+- If this snippet was placed on a page at `<your_wiki>/documentation/related/content`,
+  it would link to `<your_wiki>/documentation/main.md`:
 
 	```markdown
 	[Link to Related Page](../main.md)
 	```
 
-### Root Link
+### Root link
 
 A link starting with a `/` is relative to the wiki root.
 
-1. This snippet links to `<wiki_root>/documentation`
+- This snippet links to `<wiki_root>/documentation`:
 
 	```markdown
 	[Link to Related Page](/documentation)
 	```
 
-1. This snippet links to `<wiki_root>/miscellaneous.md`
+- This snippet links to `<wiki_root>/miscellaneous.md`:
 
 	```markdown
 	[Link to Related Page](/miscellaneous.md)