Manual.md: recommend keeping systemd units

[CameronNemo]

Closes #32481

[ci skip]

Testing the changes

• I tested the changes in this PR: N/A

[Chocimier]

What are possible negative side effects?

[Anachron]

Also closes #32154

About the side effects:
Some packages do check for systemd service files on runtime and try to start/stop when being found.
So I guess if we packaged them and keep the systemd.service files those programs might cause unexpected behavior on Void.

[CameronNemo]

One scenario I had in mind was if there was some configure flag that installed the units but also changed the runtime or dependencies of the package in some way. I did not want packagers feeling like they had to install units in that case, or patch the software to get around it. It is just sort of a generic out, letting the packager know they won't get dinged if they exclude the units.

[Chocimier]

installed the units but also changed the runtime or dependencies of the package in some way

Okay, please mention this as example in manual.

[CameronNemo]

Alright added the examples.