Specify and document new method for dev installs

[mikkonie]

Looks like the current method of pip -e is deprecated and will be removed:

DEPRECATION: Legacy editable install of django-sodar-core from git+https://github.com/bihealth/sodar-core.git@ba46c14d7cb04188ab69d3bc1936e850806e934b#egg=django-sodar-core (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457

See pypa/pip#11457 for more.

[mikkonie]

It seems the easiest way to tackle this is to add --use-pep517 to examples where we use editable installs. I added this as a subtask to #1991 as it's too small a change to warrant a separate commit. Closing this issue.