Document the requirements for using Packit jobs

[lbarcziova]

• This should be placed on a separate subpage, e.g. packit.dev/rules, so that it is linkable from other places
• This document contains the needed information
• Make sure the documentation is easy to understand (mainly that the FAS verification is not needed for every job), can be in e.g.in form of a table with the requirements and links to the rules of the particular services
• Change the onboarding docs to comply with this, link the subpage from there
• Merge this in coordination with the implementation of the changes (Change the permission checks to be done per-job packit-service#1965)

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Part of packit/packit-service#1850 epic.

[majamassarini]

Still some documentation missing.