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

[Tom Honermann via cfe-commits]

tahonermann wrote:

Hi, Scott! It's nice to see you pop up here! :)

I'm a little concerned about maintenance of this documentation going forward. As you know, the Clang AST is not intended to be stable and, even though I don't expect dramatic changes to how templates are represented, it will take some effort to ensure the code and the documentation stay in sync. Do you have any thoughts regarding how we can implement an automated process to ensure the documentation is updated?

For anyone not familiar, [ded](https://github.com/smcpeak/ded) is a diagram editor that Scott developed. I've used it in the past and found it to be lightweight and easy to use.

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