Improve the documentation for retriggering of the jobs

[mfocko]

As it has been brought up some time ago, looking for the docs regarding retriggering is not very trivial. Also from some pages we redirect to the guides for setting up Packit.

In the most ideal case packit/packit-service#1668 should resolve majority of the issues, but that would also require switching all of the “comment parsers” to the argparse…

Possible options:

• Have one authoritative page describing how to retrigger every job.
• Have the retriggering instructions next to the respective jobs.
• In any case, link from guides to either.