Local docs build with links that work locally too

Item 8 from #66.

I think I see how to do this. Basically,

1. Run rustdoc
2. Run mdbook
3. Run a nasty perl script to seddery the links appropriately

And then (also in CI)

4. Run a link checker over the whole lot using the existing link checking script or a variant thereof

Item 3 means that there won't be a way for the local docs to have links to the online ones. I think that's fine but we can come up with some markup bodge if we need one.

@nickm, does this sound OK to you? If so, I'll do it. LMK whether I should just merge it, or assign it to you for review.