Fix typo in header of doc/html.md (promt → prompt)

Close #1459.
Fix typo in doc/css.md
Remove phrase about IE conditional classes
Update to Apache Server Configs v2.0.0
Update to Modernizr 2.7.0
Remove image replacement helper class `.ir`

This class was removed because it doesn't provide enough use
cases within modern web development. Cases are mostly handled
indiviually by developers.

Reference #1472.
Closes #1475.
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: Update links and reorganize `extend.md`

* Update various links.
* Remove link to the `Node build script` repository as the project is no
  longer maintained.
* Better organize the information provided in `extend.md` by rearranging
  the sections alphabetically by title, and including a table of content
  for faster navigation.
Merge remote-tracking branch 'upstream/master'