[Python][Docs] Use editable pip install invocation in the development docs for PyArrow

[jorisvandenbossche]

Currently our docs only use python setup.py build_ext -i for building inplace.
But this is not doing an actual installation, so we should update our docs to do an editable install with pip install -e . (or at least a first time).

With the latest pyproject.toml changes, skipping this installation will now also result in a failure when trying to build pyarrow directly with setup.py (e.g. #41992 (comment)).
Moreover, direct invocation of setup.py is deprecated (although there is no clear alternative in the case of setup.py build_ext, except from re-installing with pip install)