Docs: Modular docs

[seabbs]

This PR contains a more modular approch to documentation.

In particular it:

• Removes most of the contents from the README
• Adds a quick start guide to the readme, installation instructions, contributor instructions, and citation instructioons
• Renames the main vignette to getting started.
• Moves the direct scoring documentation into its own vignette
• Renames the metric detail vignette.

This PR is entirely optional but does represent my personal preference for documentation organisation. In particular the current duplication of documentation seems like it will be dififucult to keep track of.

[nikosbosse]

Looks good to me, thank you! It would be nice to link to the pkgdown online vignettes once they are up. By the way: how do you test the pkgdown stuff when it's sitting on a random branch, rather than on main?

[seabbs]

Aha I yolo. It would be pretty easy to change up the action to upload the website as an artefact for checking like this but I haven't seen it in the wild.