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

[Aaron Ballman via cfe-commits]

================
@@ -30,6 +30,9 @@ understand some terms and definitions for readers who are not familiar with the
 C++ feature. This document is not a tutorial on C++; it only introduces
 necessary concepts to better understand use of modules in a project.
 
+A high level overview of modules can be found on the
+`Modules (C++) <https://en.wikipedia.org/wiki/Modules_(C++)>`_ page on Wikipedia.
+
----------------
AaronBallman wrote:

```suggestion
necessary concepts to better understand use of modules in a project. `Other
<https://en.cppreference.com/w/cpp/language/modules.html>`_ `resources
<https://en.wikipedia.org/wiki/Modules_(C++)>`_ can provide more background
information about modules if needed.
```
I think it's reasonable to point folks to other resources so long as they're trustworthy. I think both Wikipedia and cppreference are trustworthy and having read them both, I think they each provide a bit of value. So I suggest linking both.

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