Document artifact installation behavior per type

Users mistook --repository-file for --repository and expected it to install packages. The docs also claimed install: false works for every artifact type, while the regular pipeline ignores it for repository artifacts and repository-file artifacts never install anything.

• describe what install: true and install: false do for each artifact type
• explain the difference between --repository and --repository-file
• describe how the multihost pipeline handles artifacts via the tmt artifact prepare plugin
• mention package downgrades, the package threshold for repository artifacts and the missing reboot after installation
• drop the install=false examples for repository and repository-file
• fix the 404 links to the guest-setup tasks in the test process page

The FAQ entry on testing a newly installed kernel lives in !293 (merged).

Generated-by: Claude Code