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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

_if_ we wanted to link external documentation (we usually do not), https://en.cppreference.com/w/cpp/language/modules.html would be a better resource

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