nixos/prosody: add NixOS manual entry

We add a Prosody entry to the NixOS manual showing how to setup a
basic XEP-0423 compliant Prosody service. This example also showcase
how to generate the associated ACME certificates.

Note: The <programlisting> body might look poorly indented, but trust
me, it's necessary. If we try to increase their indentation level, the
HTML output will end up containing a lot of unecesseray heading spaces
breaking the formatting...