Fix broken links in the Synapse documentation. (#14744)

* Fix stale external links

* Fix some internal links

* Fix URLs without trailing / where needed

* Fix more links

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

* Reapply docs/openid.md fix after conflict

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

docs/workers.md

@@ -157,7 +157,7 @@ Finally, you need to start your worker processes. This can be done with either
 `synctl` or your distribution's preferred service manager such as `systemd`. We
 recommend the use of `systemd` where available: for information on setting up
 `systemd` to start synapse workers, see
-[Systemd with Workers](systemd-with-workers). To use `synctl`, see
+[Systemd with Workers](systemd-with-workers/). To use `synctl`, see
 [Using synctl with Workers](synctl_workers.md).
 
 
@@ -386,7 +386,7 @@ so. It will then pass those events over HTTP replication to any configured event
 persisters (or the main process if none are configured).
 
 Note that `event_creator`s and `event_persister`s are implemented using the same
-[`synapse.app.generic_worker`](#synapse.app.generic_worker).
+[`synapse.app.generic_worker`](#synapseappgeneric_worker).
 
 An example [`stream_writers`](usage/configuration/config_documentation.md#stream_writers)
 configuration with multiple writers: