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

[Scott McPeak via cfe-commits]

smcpeak wrote:

And more generally, is there a preferred overall approach to documenting AST structures?  My basic idea was to follow the example set by the [The AST Library](https://clang.llvm.org/docs/InternalsManual.html#the-ast-library) in the Internals Manual, but expanded to cover templates.  I'm open to doing something completely different, for example primarily or exclusively adding Doxygen comments.  The goal is simply to make it easier for someone new to learn the ASTs used to represent templates.

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