Split out the Extension docs into its own guide.

Review Request #7595 — Created Aug. 16, 2015 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x

Reviewers

The Extension documentation was getting complex, and didn't play well
with Sphinx autodoc, due to the usage of sections. This change splits it
out into a new Writing Extensions guide.

Built the docs and read through it. Links and formatting looked correct.