Update CONTRIBUTING.md's build system docs

[Mark-Simulacrum]

It seems likely that there are a number of tasks here, but the known one is that #47460 needs to be added upon merge.

[pietroalbini]

Shouldn't the forge also be updated?

https://forge.rust-lang.org/x-py.html

[steveklabnik]

Triage: it's been a long while, and this ticket doesn't have specific criteria for closing. @Mark-Simulacrum do you think this one is done, or do you know specific tasks still needed to be completed?

[Mark-Simulacrum]

I'm going to say that we should close this and if there is more work to be done (as is plausible, at least) directly target it with specific issues/PRs.