[clang] [clang][doc] Add documentation for the ASTs used to represent C++ templates (PR #66436)

[Erich Keane via cfe-commits]

erichkeane wrote:

> TBH, not sure if the UML diagrams would really help anyone understand what's going on.

I tend to agree with this, the documentation via these diagrams is confusing me as well, it doesn't really seem to add anything (and in fact, seem to be 'subtracting' in my case).

That said, if we DO decide to do this, we shouldn't commit the PNG, we need to add building of these from the script(.ded file) to cmake.

https://github.com/llvm/llvm-project/pull/66436