BuildX is required for building this container image.

There are no arm64 images published.. yet

This makes it consistent with the rest of the playbook:
- there's a default config which has various variables controlling
  settings
- there's also an `_extension_yaml` variable, which lets you override it

We disable it via `group_vars/matrix_servers`, as we do for all other
roles.

Added matrix_nginx_proxy_https_enabled to the own-nginx documentation

`always` is an nginx suffix, which shouldn't have been added to these.

Add `matrix_playbook_ssl_enabled: true` to configuring-playbook-own-webserver.md

Otherwise the public hookshot url will use http and not https

Continuation of 6cda711

Update ntfy v2.0.0-1 -> v2.0.1-0

remove comment regarding nginx in traefik doc

The newly extracted role also has native Traefik support,
so we no longer need to rely on `matrix-nginx-proxy` for
reverse-proxying to Ntfy.

The new role uses port `80` inside the container (not `8080`, like
before), because that's the default assumption of the officially
published container image. Using a custom port (like `8080`), means the
default healthcheck command (which hardcodes port `80`) doesn't work.
Instead of fiddling to override the healthcheck command, we've decided
to stick to the default port instead. This only affects the
inside-the-container port, not any external ports.

The new role also supports adding the network ranges of the container's
multiple additional networks as "exempt hosts". Previously, only one
network's address range was added to "exempt hosts".

Update ntfy 1.31.0 -> 2.0.0

Update mautrix-whatsapp 0.8.1 -> 0.8.2

Update mautrix-discord 0.1.0 -> 0.1.1

This also updates `docs/configuring-playbook-own-webserver.md`,
trying to make it up-to-date with all recent changes.

Update mautrix slack docs