Improve dev doc for running Ecole

[AntoinePrv]

Pull request checklist

• [x] I have opened an issue to discuss the proposed changes: Fix #269.
• [x] ~I have modified/added tests to cover the new changes/features.~
• [x] I have modified/added the documentation to cover the new changes/features.
• [x] I have ran the tests, checks, and code formatters.

Proposed implementation

The PYTHONPATH example is set with the default directory changed in #267 .

[AntoinePrv]

I wonder, is it also possible to install the compiled binaries permanently using pip, instead of setting PYTHONPATH?

Yes, this is the part in the installation instructions:

If all dependencies (build time such as CMake and compiler, and run time such as NumPy) are already installed, as is the case when developping Ecole, one can install Ecole with:

python -m pip install --no-deps --no-build-isolation [ecole | git+https://github.com/ds4dm/ecole | .]

So in your case

python -m pip install --no-deps --no-build-isolation .