bootstrap: doc: document std crates one by one

[Fabian-Gruenbichler]

Since switching over from individual, per crate cargo rustdoc invocations to a single cargo doc invocation for all crates, the doc output for Std became racy. Dependencies between library/ workspace crates were handled in a flaky fashion, sometimes being emitted as relative references, sometimes as references to the html root.

This is probably only a workaround, and it should actually be fixed in cargo or rustdoc, but I haven't managed to reproduce this issue outside of the Std doc build via bootstrap.

Closes: #156567

Fixes: 58e18dd

[rustbot]

r? @Mark-Simulacrum

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[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] serde_derive test:false 3.829
error: unnecessary use of `clone` to create a slice from a reference
   --> src/bootstrap/src/core/build_steps/doc.rs:814:57
    |
814 |         compile::std_cargo(builder, target, &mut cargo, &[krate.clone()]);
    |                                                         ^^^^^^^^^^^^^^^^ help: try: `std::slice::from_ref(&krate)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cloned_ref_to_slice_refs
    = note: `-D clippy::cloned-ref-to-slice-refs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::cloned_ref_to_slice_refs)]`