[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

TODO:

• Find a solution for including the C++ API. Currently a C++ handler for mkdocs-material is not implemented and the C handler is paid. A placeholder page has been added.