Implement MEP0002

[rowanc1]

We have recently accepted MEP0002 as a team:
https://mep.myst-tools.org/en/latest/meps/mep-0002/

This discusses project cross-references using markdown links, the final part of the process is to update the myst spec to include those changes. This should include at least the following changes:

• bring the content from the MEP0002 and adapt it to the spec for this page: https://myst-tools.org/docs/spec/references
  • Adaptions probably include removing some of the preamble and forward looking discussion in the MEP, and pulling in the relevant core pieces (various schemes, ways to cross reference things, etc.)
• Updating the tests and examples (testing in at least mystjs, which uses these tests today)
• Documenting the warnings (hasn't yet been done in myst-spec)
• Update the AST: https://mep.myst-tools.org/en/latest/meps/mep-0002/#specification-ast
  • Including scheme name, urlSource, internal, and updating the kind
• Removing the %s from the template/spec and only using {number}
• Deprioritizing the {eq}, {ref}, etc. roles in the spec documentation (e.g. moving to the bottom of the page)

I am happy to take a first crack at this (probably late next week), I think a lot of the pieces should be very fast to do as all the content is already there!