[libc++] Add a CMake target to re-generate files and revamp CONTRIBUTING.rst

As we automate more and more things in the library, it becomes useful for
contributors to have a single target for running all the automation as
part of their workflow. This commit adds a new `libcxx-generate-files`
target that should re-generate all the auto-generated files in the library.

As a fly-by, I also revamped the documentation on Contributing to account
for this new target and present it as a bullet list of things to check
before committing. I also added a few things that are often overlooked
to that list, such as updating the synopsis and the status files.

Differential Revision: https://reviews.llvm.org/D106067