Tracking Issue for mergeable rustdoc cross-crate info

[camelid]

This is a tracking issue for the RFC "Mergeable rustdoc cross-crate info" (rust-lang/rfcs#3662).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
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Steps

• Implement the RFC (rustdoc rfc#3662 changes under unstable flags #129337)
• Add documentation (to Rustdoc Book) -- cc @EtomicBomb
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Consider removing --merge=shared and only allow it by not providing a --merge flag. This would allow removing the implementation of shared CCI later, once Cargo uses mergeable CCI. Mergeable rustdoc cross-crate info rfcs#3662 (comment)

Implementation history

• rustdoc rfc#3662 changes under unstable flags #129337
• modularize rustdoc's write_shared #128252
• nested aux-build in tests/rustdoc/ tests #128161