Add basic documentation and auto-publish to GitHub Pages

[alexdewar]

In addition to API documentation (#15), we also want long-form documentation, including tutorials etc. as we do for MUSE v1.

There are lots of choices in terms of tools, but I think the two obvious ones are:

• Sphinx -- which we're already using for MUSE v1 and uses the RST format for files
• Jekyll -- the standard tool used for GitHub pages which uses the markdown format

@ahawkes do you have any preferences?

@dalonsoa any ideas?

[dalonsoa]

There's a rust specific option, mdBook: https://rust-lang.github.io/mdBook/index.html . There's even an GH action to deploy it to GitHub Pages automatically. This would diverge from what MUSE v1 uses, but might be the right call. I've no idea how well this integrates with the API documentation created via rustdoc.

On the other hand, rustdoc also work on plain Markdown files, so maybe that is all we need...?

[ahawkes]

I don't know the pros and cons here, so fine with me if you decide the best route.

[alexdewar]

mdBook looks good. That's got my vote! If no one has strong feelings, then we can just do it with that.