phd: add systemd unit files
Summary:
The documentation for many years has included a sample systemd unit for phd.
Save administrators some time and hassle by providing unit files and directing
them to use `systemctl link` to make use of it. This has the benefit of being a
bit faster to configure, and allows updates of the unit file at the same time
as the rest of Phorge.
Test Plan:
Try out the instructions in Diviner and observe a working phd service.
One instance from my multi-tenant install:
```
● phd@spookygirl.service - Phorge Daemons
Loaded: loaded (/etc/systemd/system/phd@spookygirl.service; enabled; preset: disabled)
Active: active (running) since Sat 2025-07-12 19:20:10 UTC; 4 days ago
Invocation: 88cc57a8c0f34b42a5616885bfff8709
Docs: https://we.phorge.it/book/phorge/article/managing_daemons/
Process: 3482251 ExecStart=/srv/phorge/bin/phd start (code=exited, status=0/SUCCESS)
Main PID: 3482256 (php)
Tasks: 1 (limit: 9510)
Memory: 24.7M (peak: 169.8M)
CPU: 1h 28min 39.112s
CGroup: /system.slice/system-phd.slice/phd@spookygirl.service
└─3482256 php /srv/phorge/scripts/daemon/phd-daemon -l spookygirl
Jul 12 19:20:09 phorge phd[3482251]: Freeing active task leases...
Jul 12 19:20:09 phorge phd[3482251]: Freed 0 task leases.
Jul 12 19:20:09 phorge phd[3482251]: Launching daemons:
Jul 12 19:20:09 phorge phd[3482251]: (Logs will appear in "/var/log/phorge/sgb/daemons.log".)
Jul 12 19:20:09 phorge phd[3482251]: (Pool: 1) PhabricatorRepositoryPullLocalDaemon
Jul 12 19:20:09 phorge phd[3482251]: (Pool: 1) PhabricatorTriggerDaemon
Jul 12 19:20:09 phorge phd[3482251]: (Pool: 1) PhabricatorFactDaemon
Jul 12 19:20:09 phorge phd[3482251]: (Pool: 16) PhabricatorTaskmasterDaemon
Jul 12 19:20:10 phorge phd[3482251]: Done.
Jul 12 19:20:10 phorge systemd[1]: Started Phorge Daemons.
```
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26181