Move backward compatibility docs under Contributors Guide

[mkaz]

Description

Closes #37510

The backward compatibility and deprecations documents were under the how to guide, but these docs do not relate to a specific how to and are relevant for contributors to Gutenberg, whereas the How to guides are targeting extenders.

The deprecations document seems to be out of date and probably a target for removal. I don't know enough if it is being used anymore or if the changelog and publishing process has replaced it. The last update to the document was for 11.0 which was 12 releases ago.

How has this been tested?

Confirm links work as expected. Since the slugs for the documents did not change, WP will handle the redirect.

Types of changes

Documentation.

• Move backward-compatibility and deprecations docs under contributors/code
• Update table of contents and manifest to new locations
• Update link to backward-compatibility document in components contributing

[carolinan]

I agree, it is nice to have a list of deprecations so that you do not have to browse through massive changelogs.
But unless the list can be automated it may be better to remove it, as it can be mistaken for a complete list of deprecations.

[ryanwelcher]

LGTM! 👍