Docs: Move Backward Compatibility to Explanations, combine Meta Boxes #37510

The how to guide section has a section for Backward Compatibility, including a list of project deprecations. This document is not showing how to do something but explains the project philosophy and history of changes. So, it should be moved under the explanations section.

Additionally, there is a "Meta Boxes" document nested under this section that is duplicated in another document in the How to Guide section, these two documents should be combined into one.