Python: Include custom static files in generated Python docs

[busunkim96]

Allow extra static files in the generated docs so the Python 2 sunset banner can be added.

See googleapis/google-cloud-python#9036

[busunkim96]

I have reached the end of my copy-pasta journey. @software-dov would you be able to take a look?

[software-dov]

I feel like I'm the least qualified person to take a look at this review; I'm not great with the monolith, and I've looked at CSS classes for maybe 20 minutes of my life.

The screenshot looks great, and that's good enough for me.

[busunkim96]

Chris pointed out that we can probably do most of this with synthtool templates. I'lll back out most of the changes here and just keep the change to docs/conf.py.

[codecov]

Codecov Report

Merging #3020 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #3020   +/-   ##
========================================
  Coverage      87.1%   87.1%           
  Complexity     5952    5952           
========================================
  Files           491     491           
  Lines         23699   23699           
  Branches       2548    2548           
========================================
  Hits          20642   20642           
  Misses         2210    2210           
  Partials        847     847

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