Split “CGB registers” into multiple pages

Currently, that page is a bit of a hodgepodge of various topics, and its contents are not easily discoverable. Following #670, I think it would be good to split that page into multiple pages grouped under a “Game Boy Color” ToC section. Note that it would be especially useful to add a “Summary” page giving an overview of the functionality, in particular introducing the terms “Non-CGB Mode” and “CGB Mode” which are used throughout Pan Docs but (as far as I know) never actually explained.

A problem is that this reorganisation suffers from the same URL breakage as #672, so I would consider consider this blocked by that issue.

Start by reviewing the current “CGB registers” page and the URL-breakage work tracked in #672. Split the material into pages under a “Game Boy Color” ToC section, add a Summary page explaining “Non-CGB Mode” and “CGB Mode,” and verify that the reorganisation does not break URLs once the dependency is resolved.

Written by the indexing model from the issue text.