[docs][coroutines] Revamp "Debugging C++ coroutines"

This commit is a major overhaul of the documentation on debugging C++
coroutines with the following goals:

* Make it more accessible to casual C++ programmers, i.e. non-toolchain
  developers. Move the low-level details around ABI further down, and
  instead start with real-life examples and copy-paste-friendly code,
  first.
* Cover LLDB in addition to GCC. Provide copy-pasteable scripts for LLDB
  and not only GCC.
* Cover additional topics, such as:
  * single-stepping into a coroutine
  * using `__builtin_return_address` for tracking suspension points
    (inspired by Folly's blog series on coroutine debugging)
* Document LLDB's support for devirtualization of `std::coroutine_handle`,
  both from an end user perspective as well as its internal implementation

Author: vogelsgesang