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

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. https://github.com/apache/arrow/issues/41992#issuecomment-2151338952).
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`)