Cross-referencing foreign custom directives

[AA-Turner]

As @chrisjsewell observed in #12536 (comment), there is no way to reference things in intersphinx objects.inv unless you have the object/extension loaded in your local build.

This doesn't seem ideal -- but fixing would require serialising much more data, on first thought. Capturing this issue for more discussion.

A

[chrisjsewell]

To note, I do already mention this here 😄 https://github.com/orgs/sphinx-doc/discussions/12152