Document the new options of `/packit test` command and job labels

[nforro]

Document the work done in https://github.com/packit/packit-service/pull/2079, https://github.com/packit/packit/pull/1988 and https://github.com/packit/packit-service/pull/2088.

[lbarcziova]

The --identifier functionality is covered by #659

[lachmanfrantisek]

Since these features are really awesome, what about:

• [x] updating the documentation (go through the docs and make it visible for all the jobs, not only tests which should be done)
• [x] copy/link the blog that Strimzi people prepared for developers.rh.com
  • [x] share to Mastodon
• [ ] (optionally) recording a demo
  • [ ] share on Mastodon

I would like to also collect some data on how many resources this can save...;)