Add example systemd unit for sshd in Diffusion docs

Summary:
The docs don't specify how to make the second sshd daemon start on every reboot.

I've added a unit that I use in my setup to the docs.

Ref T15467

Test Plan: Check that the docs look alright.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15467

Differential Revision: https://we.phorge.it/D25497