chore: publish docs to googleapis.dev

[busunkim96]

Closes #137

Removes the existing infrastructure to generate the docs and uses the standard template/format in other client libraries.

[tseaver]

The doctests build failed:

Running session doctest
Creating virtual environment (virtualenv) using python3.8 in .nox/doctest
cd /tmpfs/src/github/google-resumable-media-python
pip install sphinx sphinx_rtd_theme sphinx-docstring-typing >= 0.0.3 mock google-auth >= 0.10.0
pip install -e .[requests]
sphinx-build -W -b doctest -d docs_build/build/doctrees docs_build docs_build/doctest

Application error:
config directory doesn't contain a conf.py file (docs_build)
Command sphinx-build -W -b doctest -d docs_build/build/doctrees docs_build docs_build/doctest failed with exit code 2
Session doctest failed.

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[tseaver]

@googlebot I consent.