Set up relengapi docs to build from data_files, rather than package_data

This works great for *installed* code, but fails completely for `python
setup.py develop` installs, which go looking for the docs at `sys.prefix
/ 'relengapi-docs'`, but it's not there.
Handle development builds of the docs, too
update docs
update validate to handle docs
validate with build-docs --develop
Merge djmitche/build-relengapi:issue83 (PR #83)

Conflicts:
	base/relengapi/blueprints/docs/__init__.py