🔗 Enable external MyST project cross-references

[fwkoch]

This PR adds support for cross-references to external MyST projects, similar to intersphinx.

To do this, MyST projects serve a myst.xref.json file which contains all the available project targets. Then, when you add a reference to this project in your project config:

project:
  references:
    key: https://external-myst-project.org
  ...

you may add cross-reference links with the format [](xref:key/path#identifier).

This PR also:

• updates intersphinx to use the same syntax xref:... instead of myst:... (old syntax generates a warning)
• adds label to page frontmatter for specifying an identifier for the page that is not just the filename/slug
  • name is now an alias of label on page frontmatter
  • name is removed from project frontmatter
• warns on duplicate identifiers within a MyST project (we previously only warned on duplicate identifiers within a page)
• checks all links in your MyST project to make sure they all have link text (if a link has no text, nothing will be clickable in your site - nice to surface this issue to the author)

See also:

• Reference external MyST sites with labels and standard reference syntax (xref) #1111

TODO:

• Support these xrefs with {embed} (separate PR)
• Get project title instead of index page title from <xref:key> (separate PR)
• Replace default xref text for link of format <xref:key#id>
• Deprecate identifier on crossReference, etc nodes in favour of target (separate PR)
• Tidy up myst.xref.json - no duplicate identifier/html_id, no page slug identifiers
  • keep page identifiers though, if they are explicit!
• Better docs!

[choldgraf]

A few quick thoughts on improving the clarity of the docs here. This is looking super cool!

[choldgraf]

Just noting that the deploy preview on Netlify seems borked:

Noting here in case something in this PR has added a bug to the static HTML export...

[rowanc1]

Turns out that the myst build --html is actually currently broken. Something to look into!

[rowanc1]

@fwkoch I have moved the outstanding task list here:

• Reference external MyST sites with labels and standard reference syntax (xref) #1111 (comment)