[docs] Add document "Debugging C++ Coroutines"

Previously in D99179, I tried to construct debug information for
coroutine frames in the middle end to enhance the debugability for
coroutines. But I forget to add ReleaseNotes to hint people and
documents to help people to use. My bad. @probinson revealed this in
https://github.com/llvm/llvm-project/issues/55916.

So I try to add the use document now.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D127626