Fix issues in the structuring of cleanup-related documentation

[Martchus]

• Fix target of link "cleanup-related properties"; the anchor has been renamed to basic_cleanup at some point
• Reduce the "Cleanup" section in the installation guide so it only mentions the most important points and otherwise just directs the user to the "Cleanup …" section in the users guide which some information was moved to. This way the cleanup is still mentioned as configuration step for new installations but does not duplicate information already present elsewhere.
• Rename the "Asset cleanup" section in the users guide to just "Cleanup" and move it out of "Asset handling". This makes more sense because this section is not covering only asset cleanup.
• Remove the reference to "Use of the REST API" as this section doesn't contain related information anymore and is also not under the "Asset handling" section anymore (as it was stated).
• Related ticket: https://progress.opensuse.org/issues/129412

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching docs/*.asciidoc:

• Consider generating documentation locally to verify it is rendered correctly using tools/generate-htmldoc

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