Docs: Update server configs links

* Replace links to the Apache Server Configs `doc`
  directory, as it no longer exists:
  https://github.com/h5bp/server-configs-apache/commit/4f778c4a22da91721bc37c2eb10989b902ac4429.

* Provide missing links under the `X-UA-Compatible`
  section in the `html.md` file.
Docs: Reorganize `extend.md`

Better organize the information provided in `extend.md` by rearranging
the sections alphabetically by title, and including a table of content
for faster navigation.