fix wrong json parameter in webhook documentation

Related to: https://matrix.org/blog/2021/02/18/synapse-1-27-0-released#dropping-armv7-docker-images

Add Netlify instructions

fixes HTTP 400 Error: "{\"errcode\":\"M_MISSING_PARAM\",\"error\":\"Missing key 'avatar_url'\"}"

Remove 'sqlite3' from the Etherpad role

The official Etherpad Docker image has no support for sqlite3 databases.

Use the yaml callback plugin when running ansible-playbook

The default default Ansible screen output encodes and prints error outputs as a hard to read dense line of JSON.
This patch changes the ansible-playbook command behavior for this project to output yaml instead.

Update synapse-admin (0.6.1 -> 0.7.0)

Change the new app link to use classsic apps

Until [this issue](https://github.com/matrix-org/matrix-appservice-slack/issues/348) is fixed

Fix etherpad binding

don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used

Add additional domains for Let's Encrypt certificates to be obtained

These validation tasks should only run when Prometheus is enabled.

Prometheus and Grafana on stats.<domain>

.. and other documentation improvements.

It seems like it doesn't cause any issues for any of these services.

We mainly switch the anonymous metrics viewing variable
to false, along with other wording changes.

The quotes around "host" for both `--pid` and `--net` were
causing trouble for me:

> docker: --pid: invalid PID mode.

and:

> docker: Error response from daemon: network "host" not found.

I've also changed the `-v` call to `--mount` for consistency with the
rest of the playbook.

Fix wrong docker image being pulled

Changed `matrix_mautrix_signal_docker_image_force_pull` to `matrix_mautrix_signal_daemon_docker_image_force_pull` when force pulling the daemon

Using the hardcoded IP did break while I was
messing with IPv6 stuff on the other branch

By running it in a more privileged container with access to the host network stack and such