Improve documentation around creating extension libraries for Phabricator

Summary: There was some documentation for this but it was kind of buried in a
random, difficult-to-discover file. Separate it into its own file and link to it
from the previous location.

Test Plan: Regenerated documentation and read through it without catching
anything terrible.

Reviewers: btrahan

Reviewed By: btrahan

CC: zeeg, aran, btrahan

Maniphest Tasks: T643

Differential Revision: 1161