Add redirects for updates to block theme documentation

[mkaz]

We are setting the consistent naming of "block themes" and moving away from using the "block-based theme" name.

The documentation is also getting some clean up and reorg, so adding a few redirects for the new docs and locations.

Related to: ​https://github.com/WordPress/gutenberg/pull/31065

[mkaz]

To be implemented after:
​https://github.com/WordPress/gutenberg/pull/31167

The timing will be based on approvals of that PR, I'll wait to merge and can confirm back here when ready. Thanks!

[mkaz]

Updated diff.

[mkaz]

This one can be committed anytime, the PR was just merged. 👍

[coffee2code]

In 10925:

Developer, Block Editor: Add redirects for updates to block theme documentation.

Props mkaz.
Fixes #5714.