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

[Shafik Yaghmour via cfe-commits]

shafik wrote:

I would hate to lose the non-diagram part of this documentation. Do the criticism mainly apply to the diagrams and the text documentation is ok for most?

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