Rollup merge of #133981 - aDotInTheVoid:document-docs-ids, r=fmease

rustdoc-json: Refractor and document Id's

Closes #133780

While working on documenting Id's, I realized alot of the way they were generated was weird and unnecessary. Eg:

1. The fully uninterned id type was `(FullItemId, Option<FullItemId>)`, meaning it wasn't actually full!
2. None of the extra fields in `Option<FullItemId>` would ever be used
3. `imported_item_id` was a `rustdoc_json_types::Id` instead of a simpler `DefId`.

I believe the new implementation still covers all the same cases, but in a more principled way (and explaining why each piece is needed).

This was written to be reviewed commit-by-commit, but it might be easier to review all at once if you're not interested in tracking how the original code became the final code.

cc ``@its-the-shrimp``

r? ``@fmease``

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