Minor documentation overhaul

[mbushkov]

• Updated Sphinx. Migrated from recommonmark to myst-parser. Removed a few hack from conf.py.
• Removed most mentions of AFF4 from the docs.
• Removed Python 2-specific sections.
• Fixed the docs to suggest to change mysql server config file to set max_allowed_packet to 40M (see https://github.com/google/grr/issues/947).
• Largely rewritten "Install from source": now telling how to build Fleetspeak wheels and specifically mentioning the case when we install from source to build a client template (see https://github.com/google/grr/issues/948).
• Made minor changes to "Build custom client templates".

You can see the version of the docs from this PR at https://grr-doc.readthedocs.io/en/sphinx-update/index.html.