Add docs target and generate api docs

Once we use the pbr based build to generate files, we prune
the indexes to remove references to code we would like to
hide like the common modules and private modules in conf.py.
We reuse the same excluded_patterns variable that is already
used in Sphinx and filter these modules from autoindex.rst

Closes-Bug: #1364140

Change-Id: I5767e429e63935f055a6f4e82349224ccfa2f497
Merge "Add docs target and generate api docs"