Initial (not yet enabled) work on Heisenbridge handling media requests at matrix.DOMAIN/heisenbridge/*

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3470
- https://github.com/hifi/heisenbridge/releases/tag/v1.15.0

During testing, it appears that Heisenbridge generated media URLs
that look like this: `{media_url}/_matrix/media/v3/download/DOMAIN/FILE_ID/FILE_NAME`.

This seems off. We were expecting `{media_url}/_heisenbridge/media/something`
(e.g. `https://matrix.DOMAIN/heisenbridge/_heisenbridge/media/something`, leading to its own media proxy),
but Heisenbridge still seems to be generating URLs destined for the homeserver's Media API.

Until we figure out why that is, `media_url` remains pointed to the homeserver URL (just like before),
so that the bot can continue operating like before.

docs/configuring-playbook-bridge-heisenbridge.md

@@ -22,6 +22,8 @@ matrix_heisenbridge_owner: "@you:your-homeserver"
 matrix_heisenbridge_identd_enabled: true
 ```
 
+By default, Heisenbrdige would be exposed on the Matrix domain (`matrix.DOMAIN`, as specified in `matrix_server_fqn_matrix`) under the `/heisenbridge` path prefix. It would handle media requests there (see the [release notes for Heisenbridge v1.15.0](https://github.com/hifi/heisenbridge/releases/tag/v1.15.0)).
+
 That's it! A registration file is automatically generated during the setup phase.
 
 Setting the owner is optional as the first local user to DM `@heisenbridge:your-homeserver` will be made the owner.