Factor out the reverse proxy info to a separate file, add some more info on
reverse-proxying the federation port.

New listener resource for the federation API "openid/userinfo" endpoint

* Allow "unavailable" presence status for /sync

Closes #3772, closes #3779

Signed-off-by: Valentin Anger <valentin.an.1999@gmail.com>

* Add changelog for PR 4592

Add more tables to the list of tables which need a background update to
complete before we can upsert into them, which fixes a race against the
background updates.

A surprising number of people are using the well-known method, and are
simply copying the example configuration. This is problematic as the
example includes an explicit port, which causes inbound federation
requests to have the HTTP Host header include the port, upsetting some
reverse proxies.

Given that, we update the well-known example to be more explicit about
the various ways you can set it up, and the consequence of using an
explict port.

A surprising number of people are using the well-known method, and are
simply copying the example configuration. This is problematic as the
example includes an explicit port, which causes inbound federation
requests to have the HTTP Host header include the port, upsetting some
reverse proxies.

Given that, we update the well-known example to be more explicit about
the various ways you can set it up, and the consequence of using an
explict port.

Add ACME docs and link to it from README and INSTALL

MSC1711 certificates FAQ

Merge branch 'anoa/self_signed_upgrade' of github.com:matrix-org/synapse into anoa/self_signed_upgrade

Co-Authored-By: anoadragon453 <1342360+anoadragon453@users.noreply.github.com>