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

Allow Synapse to send registration emails + choose Synapse or an external server to handle 3pid validation (#5987)

This is a combination of a few different PRs, finally all being merged into `develop`:

* #5875 
* #5876 
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](https://github.com/matrix-org/synapse/commit/891afb57cbdf9867f2848341b29c75d6f35eef5a#diff-e591d42d30690ffb79f63bb726200891) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835 
* #5969 
* #5940

Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.

UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.

1.0 upgrade/install notes

See: https://en.wiktionary.org/wiki/successful

MSC1711 certificates FAQ

* Remove --process-dependency-links from UPGRADE.rst

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

This is based on the work done by @krombel in #2601.

* Add note to UPGRADE.rst about removing riot.im from list of trusted identity servers

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

This fallback didn't work, and was removed in #4069.

We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

This requires the psutil module, and is still opt-in based on the report_stats
config option.

I've added some Documentation, how to get the running Version of a
Synapse homeserver. This should help the HS-Owners to check whether the
Upgrade was successful.