In particular
- Adding a requirements.txt file to aid in installing docs-only
dependencies (this is a feature of readthedocs, see
https://docs.readthedocs.org/en/latest/builds.html)
- Updating comment before `import django` in docs/conf.py. Without
the setup that or tox `docs` env provides or the dependencies
in docs/requirements.txt, sphinx-build will fail due to missing
`django`
- Adding a section to docs/conf.py which downloads the
App Engine SDK and adds it to the import path
- Adding base (non-docs) dir so sources get added when generating
HTML (e.g. sys.path.append('..') within docs/ dir)
- Using package distribution for docs version instead of manually
importing oauth2client in docs/conf.py.
a39aa25091