Document how to add new branches/different Python versions to pip-compile job

[gotmax23]

          Thanks for doing this @gotmax23 I think it looks good and can see that it works from the run that you did off your branch.

I've added @samccann as a reviewer mainly as an FYI because this is something we'll need to maintain every time we cut a new branch etc. I can start documenting these things as well as the different Python versions in the README in the next week or so. We have some scribble notes but should probably convert those to something other folks can work with.

Originally posted by @oraNod in https://github.com/ansible/ansible-documentation/pull/1616#pullrequestreview-2247485538