Rollup merge of #102042 - LukeMathWalker:add-rust-json-docs-to-rustup, r=Mark-Simulacrum

Distribute rust-docs-json via rustup.

I am not 100% sure on how to treat `rust-json-docs` in `target_host_combination`. I went along with a similar strategy to the one used for `rust-docs`, but looking for guidance there.

Author: matthiaskrgr