Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

Add FAQ questions to federate.md. Add a health warning making it clear that the 1711 upgrade FAQ is now out of date.

Improved federation configuration docs.  Specifically detailing  .well-known and SRV based delegation methods. 

Inspiration Valentin Lab <valentin.lab@kalysto.org> for https://github.com/matrix-org/synapse/pull/4781