[Docs] Add a link that refers to C++ standard modules in Clang modules doc

Currently there're two pages that both talk about "Modules" in clang, but
they're different. The one that describes C++ standard modules explicitly
spells out the difference but the other one which targeting Clang modules
doesn't.

This patch adds a link that refers to the C++ standard modules
one in Clang modules doc, as you usually got the later page when
googling. I believe this will make newcomers less confused.

Signed-off-by: Jun Zhang <jun@junz.org>

Differential Revision: https://reviews.llvm.org/D134105