Are you talking about the riot-web client? There are Debian packages for it, but even if you don't use those the client is all client-side JavaScript so you can

Are you talking about the riot-web client? There are Debian packages for it, but even if you don't use those the client is all client-side JavaScript so you can statically host it. I will submit a bug report to add a quick-install guide to the README but it's quite trivial (untar into /var/www/html). matrix-synapse does have quick install guides but a RPi is probably a bit too low spec to run a matrix server that will federate with matrix.org (though it depends strongly on what rooms you join and how many users you have).