Successor of D131062. We create the new page due to the old one has too many review comments to review. So we decide to create the new page to review more clearly.
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.
I wish we could land this before September and I can backport this to 15.x. This implies that even we solve any bug during the reviewing process. We shouldn't edit the document in time. We should land it and edit it.
Tested with grammarly and make docs-clang-html.