Diviner: add a minimal systemd example

Summary:
After this change the section "Managing Daemons with phd"
mentions a minimally viable systemd configuration file
to make systemd-users happier and systemd-haters saddest.

Closes T15467

Test Plan: - read the page, see systemd, don't cry

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: 20after4, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15467

Differential Revision: https://we.phorge.it/D25293