Convert the non-CLI docs from man pages to Markdown

(Editing the top comment to be the current consensus on this issue.)

For the reasons discussed below, we've settled on Markdown as the best option for rewriting the docs.

What we'd have to do:

- [ ] Actually do the man-to-md rewrites.
- [ ] Carefully review the rendered Markdown for any regressions.
- [ ] Update rgbds-www to handle integrating the new Markdown docs into rgbds.gbdev.io!
- [ ] Update the README with the new repo directory tree. (And maybe with explanations/recommendations of how to view the rendered Markdown, since "`man man/rgbasm.5`" and `firefox docs/rgbasm.html` won't work?)
- [ ] Anything else?

====================

(Original comment follows.)

> If the website is the primary UX, I think we should have a serious discussion about not messing with these 1970s-era macros any more and just providing simple man pages that refer to HTML docs (ideally checked into rgbds, not only rgbds-www) for detailed spec info like this.

*(Originally posted by @Rangi42 in https://github.com/gbdev/rgbds/pull/1455#discussion_r1706975122.)*

Good idea!

I think the most important aspect is that there's a strong split here: the CLI references are better suited as man pages ([as supported by](https://github.com/gbdev/rgbds/pull/1455/files#r1706981737) @aaaaaa123456789); but the *language* references, not so much:

- The descriptions are more detailed, leading to rendering gripes like in the OP quoted above.
- It's hard to go to a specific anchor, `/` in `less` is annoying, and there's no ToC.
- There's a lot more content in there, and it's harder to navigate in a terminal than in a browser. (And, again, there's no ToC.)

So, I agree that it is a good idea to port *some* of the documentation away from Mandoc. (Bikeshed point: what language and tooling to use?)

One technical issue I see is how to make the pages stand-alone (for distribution, so people can keep their documentation offline) yet also transclude-able (for rgbds-www).