[docs] Add "Standard C++ Modules"

We get some standard C++ module things done in clang15.x. But we lack a
user documentation for it. The implementation of standard C++ modules
share a big part of codes with clang modules. But they have very
different semantics and user interfaces, so I think it is necessary to
add a document for Standard C++ modules. Previously, there were also
some people ask the document for standard C++ Modules and I couldn't
offer that time.

Reviewed By: iains, Mordante, h-vetinari, ruoso, dblaikie, JohelEGP,
aaronmondal

Differential Revision: https://reviews.llvm.org/D131388