Distribute documentation in JSON format via rustup for toolchain components

[LukeMathWalker]

Problem

It is currently not possible to access the JSON version of the documentation for crates bundled with the toolchain (core, alloc, std, etc.) via the same mechanism used for "normal" crates:

# This does not succeed
cargo +nightly rustdoc -p std --lib -- -Zunstable-options -wjson

This is due to the fact that those crates are pre-compiled.

Proposed solution

rustup already bundles the HTML version of the documentation for those crates.
We can start bundling the JSON version as a separate rustup component, on the nightly toolchain.

Relevant Zulip discussion.

[aDotInTheVoid]

@rustbot modify labels: +A-rustdoc-json +T-rustdoc +T-infra