Generate docs for contrib plugins

Extend write_autodoc_index to take the contrib plugins into account
and force load all resources - both built-in and contrib - in
ext/resources.py.

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Partial-Bug: #1271226
Change-Id: If93dcae52bc573e2673d7fd76a0db62d5fc68115
Merge "Generate docs for contrib plugins"