docs: add documentation

[anguillanneuf]

Fixes #44 One step closer to releasing the library!

This PR

• Exposes handwritten classes in documentation
• Adds pages for admin, publisher, subscriber clients

Question?

• Shall we publish members of google.cloud.pubsublite_v1.services and google.cloud.pubsublite_v1.types if we don't expect people to use them? I can selectively publish some but not others.

# Open generated docs in /docs/_build/html/index.html in a browser
pip install -e .
pip install sphinx alabaster recommonmark
sphinx-build -W -T -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/

# Alternatively,
nox -s docs

[dpcollins-google]

Approved once comments are resolved