Move the extensions documentation into the user manual.

A lot of people who are interested in writing extensions aren't seeing this,
because it's currently buried in the "codebase documentation". Since extensions
aren't part of the codebase per se, this should be part of the user manual
instead.

Testing done:
* Built the user manual html and checked that I could see the extensions
  documentation and that links all worked.
* Built the codebase html and saw that extensions were gone.

Reviewed at https://reviews.reviewboard.org/r/4728/