This introduces a standardized set of Spinx extensions for use in our
documentation. We have several projects today that copy/paste the same
set of extensions, and maintaining these is increasingly hard. To help
keep this under control, they've been pulled out into this centralized
package.

Later, we'll be able to add additional tools or extensions to help with
our doc building processes. For instance, bundling newer (or forked)
releases of napolean.

The extensions in here are taken from Djblets, but have had some
enhancements made. Each has had documentation added to the source files,
and a couple of extensions have had updates:

* The github linking extension has been updated to be more generic,
  rather than hard-coding GitHub org/repo names and versioning logic.
* The autodoc_utils extension now supports a __autodoc_excludes__ list
  in modules.

This is all documented within the individual files and within the
README.rst.

Switched Djblets over to this and built the docs. Everything worked.
Tested that GitHub linking and module-level exclusions also worked.