doc: Add debugging tips?

Doing a quick search of "debugging" doesn't bring up too much in the pybind11 docs.

For the repo that I work on, we've found it useful to document some debugging tips that are relevant to Python C extensions:
https://drake.mit.edu/python_bindings.html#debugging-with-the-python-bindings

Perhaps some mention of using trace, perhaps a small example with GDB would go a long ways?
For GDB, it'd be nice to show (a) debugging a module and (b) debugging a C++ binary with an embedded interpreter.
I find that for hacking directly on pybind11 (in things like #1325 and #1146), it's uber useful to plop into Python via an embedded interpreter, rather than debugging through the python executable itsefl.

Search the pybind11 documentation for existing debugging coverage, then read the linked Drake Python bindings debugging page for relevant examples. Define the documentation scope around trace usage and GDB examples for both a Python module and a C++ binary with an embedded interpreter; done means these scenarios are clearly documented.