docs(installation): add systemd service

[cloudrootab]

Description

I could never get the yarn forever service variant to work. However, it was quite trivial to make a systemd variant on both my test-systems. So I've decided to share those steps that I did. Hopefully you'll find it usefull/complementary.

This adds an alternate subsection with examples on what files to create and what commands to execute to have it being a systemd service instead..

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft