[clang] [clang][docs] Add link to C++ modules Wikipedia page to docs (PR #169200)

[via cfe-commits]

mikomikotaishi wrote:

If there is concern that its location on the page gives too much priority to it, I alternatively propose to put it in a "further reading" section, similar to [Debugging C++ Coroutines](https://clang.llvm.org/docs/DebuggingCoroutines.html) page.

https://github.com/llvm/llvm-project/pull/169200