Generate Docs with Sphinx

[nosracd]

This PR builds the documentation via Sphinx:

• docs/content/mainpage.md has been converted to docs/index.rst
• Other markdown documents are processed using myst
• To make the markdown docs compatible with myst, this PR also:
  • Removes explicit cross references
  • Converts tables to markdown format
  • Limits to one top-level section per document
  • Replaces \section, \ref, \c, \li, \include, \code directives
• epydoc only supports python 2, so the python API is now generated via sphinx

[nosracd]

• The previous link to the Python docs was http://lcm-proj.github.io/lcm/python/index.html. I'm not sure if we can but it might be a nice-to-have to provide a redirect to the new Python doc location.

I ended up just renaming python_api.rst to python/index.rst to be consistent with the previous location.

• The copyright is showing up as Copyright .. Would it be possible to either fill in a year or remove it completely?

Apparently there's a html_show_copyright setting so I just set that to False