mk: Stop documenating non-facade crates

This commit ceases documentation-by-default of crates such as `term`,
`serialize`, and `alloc`. Crates like `term` and `rand` have duplicates on
`crates.io` and the search index entries generated in the local tree end up
only leading to confusion. Crates like the entire compiler infrastructure,
`flate`, or `rbml` don't need to be documented in such a prominent location.

This change also means that doc tests will no longer be run for crates beyond
the facade (e.g. `serialize` or `term`), but there were very few doc tests in
there to begin with.

Closes #22168

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