Update sliding sync doc for external DB users

* fixes #2586

fix(sliding-sync): Use external syncv3 server URL

It should be noted that this cannot be used for the initial install
of services which require a database or have other dependencies.
Those would typically need to invoke the playbook with
`--tags=install-postgres,install-SERVICE`, etc.

The purpose of this shortcut is to easily rebuild and restart
a single serice subsequently. For those cases, often times there's no need to
reinitialize the database and other components and simply running a
single component's tasks is enough.

update ntfy 2.1.2 -> 2.2.0

migrate prometheus-node-exporter's var

add database for Dendrite's relay API - fixes #2571

i was sleepy lol

Update hookshot 2.7.0 -> 3.0.0

Update mautrix-whatsapp 0.8.2 -> 0.8.3

Update mautrix-discord 0.1.1 -> 0.2.0

Warning! Potentially breaking change: https://github.com/matrix-org/matrix-hookshot/releases/tag/3.0.0

For backward-compatibility reasons, this playbook uses `matrix` as its
main and default database.

Update borgmatic 1.7.8 -> 1.7.9

It wasn't used at all. It doesn't seem like there's a way to control
the log level, judging by:
- 8f9eea4114/.env.example
- 8f9eea4114/src/env.ts

Update element 1.11.24 -> 1.11.25

fix typo in dendrite dynamic thumbnail var

Related to:

- 682d31c702
- ecbe0ddbe7
- f4fc83ac75

.. and https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2569

Upgrade Synapse from 1.78.0 to 1.79.0

Related to:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2565
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2566

Update dendrite to v0.12.0

add dynamic thumbnails var for Dendrite

add full text search for dendrite

add synapse_auto_compressor