Enhance documentation for python venv creation

Docs are now far better regarding python and especially the part "Create an healthy Python environment for OTB".
But I believe there is an issue with the pip install command example.
It is suggested to install geopandas first, and without the --no-binary option.
Thus shapely will be installed, and it ships with its own version of libgeos. It will generate an issue for sure, in case of version mismatch.

Today I tried to compile both shapely and rasterio and ran into errors, since the build backend for shapely (meson / ninja) does not make use of CMAKE_PREFIX_PATH so ninja was missing OTB include dir.

For the record, first I tried using pip install --config-settings which is supposed to allow passing arguments to the build backend but I couldn't get ninja to build shapely with OTB lib/ (using -Csetup-args=--libdir), whereas these two variables did the trick:

export CFLAGS="-I$OTB_INSTALL_DIR/include"
export LDFLAGS="-L$OTB_INSTALL_DIR/lib"

It would be great to add it in otbenv.profile, or at least in the docs (e.g. adding those exports to bin/activate).

Edit: it turns out I needed that because the geos-config command isn't working as expected (returning wrong paths). So I opened #2461 (closed).

Then, after installing numpy and recompile_bindings.sh, a recommended pip install command would be :

pip install rasterio pyogrio pyproj shapely --no-binary :all:
pip install geopandas scikit-image scikit-learn

N.B. : here we compile geopandas GIS related deps (pyogrio, shapely and pyproj) but we avoid geopandas in the --no-binary cmd, because it would also trigger pandas build (and it's quite long).

I know many people had this libgeos version error with geopandas, and @remicress ran into similar issues so I'm tagging you here !

Also while I'm here, another suggestion to make "activate otb" simpler :

echo "source $OTB_INSTALL_DIR/otbenv.profile" >> otb_venv/bin/activate