We should document the recommended approach for packages that change from one build system to another.

Rationale

I get asked this question almost weekly (#13466, #19298, #20364, #22209, #25111). There have been several attempts to better handle packages with multiple build systems (#10411, #12941, #15420), but we have yet to reach a consensus on any of them. We might as well document the currently recommended approach.

Description

We should add a table similar to #12941 (comment) showing good examples of packages that have changed build systems. The current recommended approach is to inherit from the new build system and override phases using @when decorators as needed, then deprecating the old versions so they can be removed in a future Spack release.