🔗 Support project cross references in embed

[fwkoch]

Previously, project cross references were enabled with [](xref:key#identifier). Now, you can also embed this content with

:::{embed} xref:key#identifier
:::

This content gets modified to indicate that the source location has a different base URL, so links/xrefs/etc resolve correctly.

This PR includes a number of other related improvements:

• project cross references allow cite style, e.g. @xref:key#identifier
  • This required some reordering and reorganization of link/cross-reference resolution
• slightly better caching - centralized logic, maxAge - this de-risks the reordered link resolution a bit (we run the link transforms more often, but results are more reliably cached)
• local cross references with page, e.g. [](page#identifier) now resolve to the same type of cross references as identifier-only, e.g. [](#identifier)
• embed nodes with page also resolve, e.g.

  :::{embed} page#identifier
  :::

  Previously those just did not work at all.

[choldgraf]

[rowanc1]

Testing now @choldgraf ... hopefully getting this out this eve! 🚀