label required version for config options, and other compatibility details

[BenTheElder]

What would you like to be documented:

https://kind.sigs.k8s.io/docs/user/configuration/ should surface what the minimum version of kind is for each config field, any differences in behavior, and the minimum apiVersion for the field.

The versions would probably look best as some sort of marker (table?) below each heading, but we should try some things to see what works best to effectively communicate all of this. Some of it should probably just be text within the details of the feature, renamed fields, etc.

Why is this needed:

To more clearly communicate support.

---

I think we should probably just start with v1alpha4, it's probably not worth it to go back and document older alphas in depth, we're not really going to support year old kind versions yet ...