Improve the userdev documentation

[Strokkur424]

I think userdev has grown big enough that it warrants more than just one page. It could also include all documentation currently "documented" in https://github.com/PaperMC/paperweight-test-plugin.

Another reason would be because the userdev page has a lot of asides, which should be split up/improved (https://github.com/PaperMC/docs/pull/681#issuecomment-3448010168)