Add support for "project-wide" cross references

[agoose77]

Proposal

Right now we only understand MyST at a single-document level. It would be useful to implement support for a MyST "project" (taking inspiration from _toc.yml etc), so that we can resolve cross references.

[rowanc1]

Very excited about this @agoose77 -- let me know how I can help or if you want to spend next monday breaking this down into a few different stages!