Add a page for unsafe precondition checks

[saethlin]

@ChrisDenton asked me to write up a little explanation about rust-lang/rust#120594 for the library team.

I really don't know what kind of writing or formatting is preferred here, but I'm happy to reshape this if people have opinions.

[ChrisDenton]

This seems good to me!

I think it really helps if we start documenting this kind of stuff as we go because I do think it helps both onboarding new contributors and in keeping existing reviewers and contributors up to date.