[Python][Docs] Document how to use Ninja to build pyarrow with PYARROW_CMAKE_GENERATOR

[jorisvandenbossche]

Using ninja instead of the default cmake generator can be quite beneficial (you get some form of cached / incremental build, although not perfect), but we don't really document this at https://arrow.apache.org/docs/dev/developers/python.html

export PYARROW_CMAKE_GENERATOR=Ninja

[jorisvandenbossche]

After using this a while, I have to say this is quite broken. Half of the time, it doesn't properly reinstall the extension modules that it should (based on which files were changed. For example, I edit _dataset.pyx, it recompiles a bunch (some subset) of cython files, and in the end sometimes prints -- Up-to-date: /home/joris/scipy/repos/arrow/python/pyarrow/./_dataset.cpython-310-x86_64-linux-gnu.so, while it obviously was not, and should have reinstalled it)