Clean up build system code that handles QfP documentation building

QtForPython uses a combination of CMake + setuptools + sphinx to drive the building of documentation.

Instructions can be found here
https://doc.qt.io/qtforpython/gettingstarted.html#building-the-documentation

python setup.py build_rst_docs is a custom setuptools command that calls cmake to configure the doc projects, bypasses calling the build tool (ninja/make) and directly calls sphinx.

The procedure above does not build Qt API docs.

To build the Qt API docs one must cd into the build dir and run ninja apidoc.
That target is generated by CMake. It is not part of ALL target, so it's not built by default when calling just 'ninja'.

There are also shiboken docs which are built by default when calling setup.py.
The custom target is called 'ninja doc' not 'ninja apidoc'.

It would be good to fix build_rst_docs to call 'ninja' instead of sphinx directly.
It woul also be good not to build shiboken docs by default to follow the pattern of Qt apidocs not being built by default.
An option could be provided to force building them as part of ALL target.
There are also a py_script.py file generated by cmake for api docs, which should be extracted into a regular python file.