Add documentation about the profile MFE

[deborahgu]

There’s no real public facing documentation of the profile MFE, aside from basic release notes & generated docstring API docs.

Conditions of Acceptance

• Determine where in our public documentation that document would go. We don't seem to have prior art for docs of any of the MFEs. Consider asking #documentation at openedX slack or discourse if there's a consensus.
• Make basic documentation that covers at a minimum
  • Which backend sources are used for profile (e.g. user API, certificates API)
  • Which backend sources get updated, potentially, based on profile MFE changes
  • how we store social links and language proficiencies
  • what the extended profile is and its relevance here