[libc++] Documents details of the pre-commit CI.

This documentation aims to make it cleare how the libc++ pre-commit CI
works. For libc++ developers and other LLVM projects whose changes can
affect libc++.

This was discusses with @aaron.ballman as a follow on some unclearities
for the Clang communitee how the libc++ pre-commit CI works.

Note some parts depend on patches under review as commented in the
documentation.

Reviewed By: ldionne, #libc, philnik

Differential Revision: https://reviews.llvm.org/D133249