Explain in docs the use of parent images once the dust has settled

[AlanCoding]

I have been putting in rolling updates to the gist here:

https://gist.github.com/AlanCoding/a61ec24e35b980dca27f5e2ddb553a1d

This was with the intent for it to eventually go into docs. However, at the moment, things may be moving too fast to effectively migrate these into documentation. After all of the other discussed rewrites are finished we should migrate the information in that gist (or its replacement) into the docs pages, either here, or for ansible-runner, or some other similar place.

Right now if you search for "python-builder" you get 1 result:

https://github.com/ansible/ansible-builder/blob/f53d6415128c918d00c046c6eba040be95cd2fa2/ansible_builder/constants.py#L18

If you search for "multi-stage" builds, you find several mentions in issues, but no mentions in code.

[shanemcd]

@AlanCoding We might be able to do this now. Removing the blocked label.