Convert project docs from mkdocs to myst-sphinx

[ianthomas23]

Convert project docs from mkdocs to myst-sphinx. This is what they look like:

emforge-docs.mp4

You can build and serve the docs locally using pixi run docs-build and pixi run docs-serve, which are the same commands as before this PR. The pins in .pixi.toml for the doc dependencies are quite tolerant, we can tighten them up later if we need to. I use myst-sphinx in many other projects so I usually discover problems quite quickly.

There is a slight change to the CI workflow that generates and uploads the docs to github pages. We can only check it works on merging this to main as that is when github pages is updated.

Once this is confirmed to work on main I will forward port to the emscripten-6x branch.