Link to documentation-specific guidelines.

Changed because it's not obvious how to get from the previously used URL to the documentation-specific content.

This is partly because the original URL was previously changed to point to different content:

 * https://github.com/rust-lang/rust/pull/74037/files#diff-242481015141f373dcb178e93cffa850L88

 * 3f6928f#diff-6a3371457528722a734f3c51d9238c13L12

library/std/src/lib.rs

@@ -85,7 +85,7 @@
 //! # Contributing changes to the documentation
 //!
 //! Check out the rust contribution guidelines [here](
-//! https://rustc-dev-guide.rust-lang.org/getting-started.html).
+//! https://rustc-dev-guide.rust-lang.org/contributing.html#writing-documentation).
 //! The source for this documentation can be found on
 //! [GitHub](https://github.com/rust-lang/rust).
 //! To contribute changes, make sure you read the guidelines first, then submit