Use pybind11_mkdoc to make Python docstrings from C++ comments

[aloisklink]

Currently, our docstrings for Python are explicitly defined as C++ std::strings, e.g. with something like:

nqm-irimager/src/nqm/irimager/irimager.cpp

Lines 75 to 79 in 88834c9

	m.doc() = R"(Optris PI and XI imager IR camera controller
	We use the IRImagerDirect SDK
	(see http://documentation.evocortex.com/libirimager2/html/index.html)
	to control these cameras.)";

However, this is pretty inconvenient as:

1. The docstrings have to be defined in the pybind11 binding code, not where the functions are defined. This is currently not too bad as everything is in one file, but we'd generally rather have a separate file for the IRImager C++ class, and then a separate file that deals with binding the C++ class to a Python class/module.
2. C++ normally expects documentation in Doxygen comments, e.g. like:
   

   nqm-irimager/src/nqm/irimager/irimager_class.hpp

   Lines 28 to 37 in 278fd06

   	/**
   	* Start video grabbing
   	*
   	* Prefer using `with irimager: ...` to automatically start/stop streaming
   	* on errors.
   	*
   	* @throws RuntimeError if streaming cannot be started, e.g. if the camera
   	* is not connected.
   	*/
   	void start_streaming() {

   
   Using Doxygen comments is a lot more familiar, especially since most code editors will have some sort of intellisense/code-previews that automatically shows the comments for a function. However, there's no way to access comments in C++, since they're part of the pre-processor.

pybind11_mkdoc is a tool that uses Clang to parse our C++ code, automatically extract the comments, and create a docstrings.h header file that contains the comments of every single function, but as a string that can be imported into C++ code.

I've followed the approach listed in pybind/pybind11_mkdoc#23 (comment) to run pybind11_mkdoc in CMake. This is required because:

• pybind11_mkdoc needs to know where to include all of the C/C++ header files, and only CMake knows where all of these library ares (e.g., if you install via pdm/poetry, pybind11 is generally saved into a temporary folder, but not if you're installing via pip3).
• pybind11_mkdoc needs to be run before C++ is run, since the files are required for compilation.

[AshleySetter]

Looks good to me 👍