Document how to check binary RPMs with Testing Farm

[TomasTomecek]

Follow up to #601

Blocked on:

• https://gitlab.com/testing-farm/general/-/issues/65
• https://issues.redhat.com/browse/TFT-1574

rpminspect allows analyzing binary RPMs and that's how it runs on dist-git. Let's document how to do this with Packit and Testing Farm once the RPM files are available in the testing environment.

(we could download them ourselves in theory but if this is what TF is meant to do, let's do it the right way)

[lbarcziova]

The latest release of TF should have unblocked this, so we can test this out and document it.

[LecrisUT]

Also, one thing to consider. How about making these tests reusable so that they can be imported? E.g.:

plan:
    import:
        url: https://github.com/packit/packit
        name: /rpmlint

The only tricky thing would be how to copy the rmplintrc file.