DOCTEAM-1553: adds topic-set targets

[Amrita42]

PR creator: Description

Adds a topic on how to set targets to the existing article Introduction to systemd Basics

PR creator: Are there any relevant issues/feature requests?

• bsc#...
• jsc#...

PR reviewer: Checklist for editorial review

Apart from the usual checks, please double-check also the following:

• do any new files fulfill the naming conventions specified in https://github.com/SUSE/doc-modular/blob/main/templates/README.md?
• article title/structure title: does it have the required length and does it use sentence style capitalization?
• revhistory: is it up-to-date and does it follow the guidelines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-revhistory?
• metadata: does it follow the guidlines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-taglist-smartdocs?

[dariavladykina]

Hi, please see some suggestions for this one. Thanks!