Document better that it's OK to use CI for testing

[jyn514]

Currently this is documented as

If your machine is not very powerful, a common strategy is to only use ./x.py check on your local machine and let the CI build test your changes when you push to a PR branch.

But it's hidden all the way down in https://rustc-dev-guide.rust-lang.org/building/prerequisites.html#hardware, which is a long way away from https://rustc-dev-guide.rust-lang.org/contributing.html#pull-requests (which is itself not easy to find).

We should

• Add a note near the 'pull request' section that Rust has plenty of CI capacity for PRs and it's fine to use it
• Make the pull request section easier to find
• Link to it from more places

For context, this is meant to be a replacement for rust-lang/rust#76272.

[ehuss]

There is now a dedicated chapter for this at https://rustc-dev-guide.rust-lang.org/tests/ci.html.
However, I didn't add links to it from the other places.

[jieyouxu]

Triage: we should still link to CI testing from contribution workflow