Add new image_xref dev doc

[matttrach]

The goal is to document all of the images we use, how they are built, what they are based on, what binaries are in them and how we build those binaries.
This is a compilation of the investigation from #4194.

[brandond]

This appears to cover a lot of the same detail as https://github.com/rancher/rke2/blob/master/developer-docs/image_sources.md which has become stale. If we're going to start a new doc instead of updating/augmenting that one, can we perhaps get rid of that old one?

Alternately, since the table format may be more easy to consult at a glance, maybe we could get the existing doc updated and cross-linked to this one?

[briandowns]

I agree. Updating the existing would probably be better and easier to read.

[matttrach]

I took your concerns into consideration and tried to clarify and separate the data as best as I could. I think this research is valuable to developers working on Rke2 in its current format and we should accept it into the codebase. I minimally altered the image_sources.md to reflect a new viewpoint on the data as well as clarify the goals and uses of the other documents. I don't have a lot more time to spend on this at the moment, so I hope we can accept this into the codebase as it is and iterate further in the future.