Couple of additional tweaks to the mjolnir docs

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/962

Add mjolnir

Add some advice about making Synapse faster

Upgrade synapse-simple-antispam (0.0.1 -> 0.0.3)

Upgrade Element (1.7.24 -> 1.7.24.1)

Also removes some duplicate table of contents entries in maintenance-synapse.md

matrix-nginx-proxy: add custom nginx options to nginx.conf.j2

Updates to mautrix-signal config

See these last commits:

tulir/mautrix-signal@4fc34330c1f6947aece67863b0d04da34c776f80

tulir/mautrix-signal@64bc5c36a509ba435a0b01cf44afb1b5d2642efd

tulir/mautrix-signal@ddda1666d41d28750cc59d070e4388b24add6ad9

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/963

This also simplifies Prerequisites, which is great.

It'd be nice if we were doing these checks in some optional manner
and reporting them as helpful messages (using
`matrix_playbook_runtime_results`), but that's more complicated.
I'd rather drop these checks completely.

Some people have reported Dimension failing to contact
matrix-nginx-proxy/Synapse on startup. This attempts to
delay Dimension startup some more.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/970

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/970

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/969

feat: push ephemeral events to appservices

This adds https://github.com/matrix-org/matrix-doc/pull/2409 to the
appservice registrations, enabling synapse to push EDUs to appservices.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/966

Update nginx (1.19.6 -> 1.19.8)

That is:
- enabled in the role by default
- disabled in the compilation (playbook), if considered an optional
component

We don't point them straight to Synapse anymore, but rather
to matrix-nginx-proxy, which forwards to Synapse (or something else).

Make ma1sd's SRV record optional

This variable was previously undefined in the role and was only getting
defined via `group_vars/matrix_servers`.

We now properly initialize it (and its good default value) in the role
itself.