Redirections from the old RGBDS docs to the new ones

Since the RGBDS repo was transferred from rednex/rgbds to gbdev/rgbds (see issue #567), the docs moved from* to (source code). The redirections in this repo are there to ensure that the old pages don’t become 404s.

In general, Rednex as an organization has been merged into gbdev, so please open issues etc. there. Thank you!