and provide an example for using the admin API

The bit about the matrix-make-user-admin script was messed up (it wasn't actually a code block so the "<username>" was hidden). For me at least it seems like the ``` syntax is much harder to accidentally mess up.

Docs for #161 (Github issue) and #168 (Github Pull Request).

The previous URL returns a 404.

Hopefully, we no longer lead people to believe that DNS SRV
records are going away forever and for all use-cases.

Fixes #156 (Github Issue)

It's too many configuration options to keep them in a single list.
Trying to put some order.

Fixes #129 (Github Issue).

Unfortunately, we rely on `service_facts`, which is only available
in Ansible >= 2.5.

There's little reason to stick to an old version such as Ansible 2.4:
- some time has passed since we've raised version requirements - it's
time to move into the future (a little bit)
- we've recently (in 82b4640072) improved the way one can run
Ansible in a Docker container

From now on, Ansible >= 2.5 is required.

Inspired by #128 (Github Issue), we've created a new Docker image
to replace https://hub.docker.com/r/qmxme/ansible

Adding dnspython or dig to `qmxme/ansible` doesn't seem like a good
idea (that might be accepted by them), given that it's specific to our
use case. That's why we'll be maintaining our own image from now on.

When using Let's Encrypt SSL certificates, a cronjob is set up to
automatically renew them. Though it does require a `cron`-compatible
program on the server.

This fixes the error that is caused by the `/etc/cron.d` directory
not existing and the `ansible-cron` module trying to write out a
file there -- without checking if the directory exists first.

Related to #108 (Github Pull Request).

It didn't mention `matrix_appservice_discord_client_id` and
`matrix_appservice_discord_bot_token`, which makes it hard for
beginners.

Related to #105 (Github Pull Request).

Bridge is setup to work on the matrix side with this, but the discord invite link is not automatically generated.

`matrix_synapse_no_tls` is now implicit, so we've gotten rid of it.

The `homeserver.yaml.j2` template has been synchronized with the
configuration generated by Synapse v0.99.1 (some new options
are present, etc.)

We had something like that on the Server Delegation how-to page,
but it's better if we have it on the SSL certificates page.

Relocated there and improved linking.

Fixes #94 (Github Issue)