[libc++][doc] Improves contribution page.

This adds more information regarding the libc++ coding style and
reference that are useful when working on a standard library
implementation.

This information is based on review comments and tips I give to new
contributors an information I wish I'd know when I started working on
libc++.

Depends on D156051

Reviewed By: #libc, jloser, var-const, ldionne

Differential Revision: https://reviews.llvm.org/D156052