docs: create a drop down with cockroachDB versions supported by the operator across the major releases

[dbist]

Reading the manifest for the supported 19.2 release is bad user experience

[johnrk-zz]

@dbist , To clarify, are you referring to having to read this operator manifest file to get a list of supported versions of CockroachDB?

[dbist]

@johnrk I think you meant to tag me and not Dan but yes, searching the repo for the manifest file instead of seeing it front and center in the docs is a bad experience.