[Docs] Use `mkdocs` for documentation

[smokestacklightnin]

Google Would like to migrate the documentation for serving away from their internal systems, so this PR implements that migration.

Changes in this PR:

• Add mkdocs to the dependencies and add a configuration for it
• Add a GitHub workflow to deploy the documentation on GitHub Pages
• Fixed admonitions
• Add white background to rendered png images so they are legible in dark mode
• Move tensorflow_serving/g3doc/ -> docs/, the standard location for documentation
• Build C++ API docs with Doxygen

Here is a preview

[peytondmurray]

@gharibian Is there anything I can do to help push this through? It would be really nice to get this merged.

[smokestacklightnin]

@gharibian Is there anything else you need us to do before this PR is merged?

CC: @peytondmurray