docs: python bindings howto
* Added new advanced section with an example of using the Python bindings with CPython code compiled back to C code using Cython. * Though it may seem a bit counter-intuitive to use the bindings just to go back to C via a different route, this is not actually stupid. * Added examples/howto/advanced/cython/ directory. * Added keycount.pyx, setup.py and the keycount.c file which the first two generated with Cython. Not including the .so and .o files from the build. * Exported the .texi version of the howto for the main docs.