Add documentation about systemd

[jimcat8]

Author checklist (Completed by original Author)

• Good fit for the Rocky Linux project? Title and Author Metatags inserted ?
• If applicable, steps and instructions have been tested to work
• Initial self-review to fix basic typos and grammar completed

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Detailed Editorial Review and Peer Review)
• Final approval (Final Review)

[sspencerwire]

While it may seem counter-intuitive, there shouldn't be a "The" at the beginning of the sentence. Simply "systemd" will suffice.

[github-actions]

Test results for e86273d:

Number of broken URLs: 4

URL,RESULT,FILENAME
 https://www.linode.com/docs/guides/beginners-guide-to-lxd-reverse-proxy,failed,guides/containers/lxd_web_servers.md
 https://google.com/books/index.html,failed,books/admin_guide/15-three-swordsmen.md
 https:%%google.com/books/index.html,failed,books/admin_guide/15-three-swordsmen.md
 https://google.com/books/index.html\ntitle//tcp,failed,books/admin_guide/15-three-swordsmen.md