Update links

[JohnTitor]

• Replace https://forge.rust-lang.org/platform-support.html with https://forge.rust-lang.org/release/platform-support.html
• Replace https://github.com/rust-lang/rustup.rs with https://github.com/rust-lang/rustup

Also,

rustup/Cargo.toml

Line 6 in 7832b2e

documentation = "https://rust-lang.github.io/rustup.rs/rustup/index.html"

should be replaced but I'm not sure what the correct link is (https://rustup.rs?).

[kinnison]

We don't really have rustdoc published any more for rustup because it's not a library for others to use. I'd suggest we either replace it with https://rustup.rs/ or just drop the documentation entry alltogether.

[JohnTitor]

I feel it's meaningless nowadays so I dropped it. (offtopic: I saw your message on Discord, no rush, take care of yourself!)

[kinnison]

All these look good to me, Thank you!