Allow rustdoc JSON rustup component to be installed on stable.

[obi1kenobi]

Currently, the rust-docs-json component is only available on nightly Rust.

It would be very useful to cargo-semver-checks to have rustdoc JSON available in stable as well, to allow performing cross-crate linting that involves types from std/core/alloc etc. This will also simplify the process of linting the standard library itself for possible breaking changes, which is something T-libs is interested in as well.

Even though rustdoc JSON is an unstable component, each JSON file carries a version number meaning that the installed files remain parseable and usable with the matching version of rustdoc-types. So even though future stable Rust releases may change the format, it still makes sense to provide this machine-readable artifact that says "this is what the standard lib contained in this stable release."

For avoidance of confusion: different stable releases may ship mutually-incompatible rustdoc format versions in the rust-docs-json component. The component's contents and shape are valid with respect only to the toolchain in which it is found, analogously to how the rustdoc HTML docs component works.

r? @Mark-Simulacrum @GuillaumeGomez

[obi1kenobi]

@rustbot label +T-rustdoc +A-rustdoc-json

[obi1kenobi]

@rustbot label -T-rustdoc

thanks Urgau! I misspelled the rustdoc JSON team label 🙈