Document the internal XMPP service
Originally created by @intrigeri on #14498 (Redmine)
The new internal XMPP service shall be briefly documented in the Services section on https://tails.boum.org/contribute/working_together/roles/sysadmins/. Drafting something should take ~10 minutes.
Feature Branch: bug/14498+document_the_internal_xmpp_service