Make the matrix domain's homepage useful

[spantaleev]

As proposed by @anadahz, we should make the homepage at matrix.DOMAIN show something useful or redirect somewhere else, instead of being a 404 page.

Redirecting to riot.DOMAIN is one option, but also an option that would fail if matrix_riot_web_enabled: false.

It would probably be ideal if we have our own simple HTML page there, which contains an introduction to Matrix and possibly a link to this playbook.

[aaronraimist]

Synapse v0.34.0rc2 adds this page where the old webclient used to be https://matrix.raim.ist/_matrix/static/, could redirect matrix.DOMAIN to this

[spantaleev]

matrix-org/synapse#4289 says:

We'll redirect to it from / if we have nothing better to display there.

.. but I'm not sure if it does it already or it will be something that's done in the future.
If they don't do it by default, we may be able to set up proxying ourselves too.