nixos/doc/matrix-synapse: refactor

* Linkify all service options used in the code-examples.
* Demonstrated the use of `riot-web.override {}`.
* Moved the example how to configure a postgresql-database for
  `matrix-synapse` to this document from the 20.03 release-notes.

nixos/doc/manual/configuration/configuration.xml

@@ -21,7 +21,6 @@
  <xi:include href="xfce.xml" />
  <xi:include href="networking.xml" />
  <xi:include href="linux-kernel.xml" />
- <xi:include href="matrix.xml" />
  <xi:include href="../generated/modules.xml" xpointer="xpointer(//section[@id='modules']/*)" />
  <xi:include href="profiles.xml" />
  <xi:include href="kubernetes.xml" />