Document github automation development setup and token requirement

[robobario]

Ah I see I've put it in the releasing pre-reqs, I was thinking of a different persona here. I went to set up my fork to test the workflow and I knew there would be some sonatype variables that need to be set, so I went to the workflow to find them.

So it feels like there's a section or link missing for how to set up your fork for github actions dev in this repo. Keeping it distinct from this doc would be nice. I'll revert the changes out and can think about it separately.

Originally posted by @robobario in #514 (comment)

This repository should have some documentation covering:

• the requirements of any github access tokens, so that when they expire we can update them correctly. Likely this can be a link to the established docs in the kroxylicious/kroxylicious repo since we have organization level secrets.
• instructions for what secrets/variables a fork needs to exercise the workflows. I'd like something that mentions the sonatype tokens, again likely linking off to the other repo for more details.