Prevent changing kernel spec name in notebooks

[soxofaan]

eec1fba added note in README to not change the kernel spec of a notebook (because that messes up the end-user experience).

I think we should enforce this better with a check script and github action

[soxofaan]

Added initial check script that detected these current issues:

Checked 11 notebooks
Found 5 kernel spec issues:
sentinelhub/process_request.ipynb has unsupported kernel 'python3' (from {'display_name': 'Python 3 (ipykernel)', 'language': 'python', 'name': 'python3'})
sentinelhub/pollution.ipynb has unsupported kernel 'python3' (from {'display_name': 'Python 3 (ipykernel)', 'language': 'python', 'name': 'python3'})
sentinelhub/migration_from_scihub_guide.ipynb has unsupported kernel 'python3' (from {'display_name': 'Python 3 (ipykernel)', 'language': 'python', 'name': 'python3'})
sentinelhub/lake_ice_monitoring.ipynb has unsupported kernel 'python3' (from {'display_name': 'Python 3 (ipykernel)', 'language': 'python', 'name': 'python3'})
openeo/NDVI_Timeseries.ipynb has unsupported kernel 'python3' (from {'display_name': 'openeo_py', 'language': 'python', 'name': 'python3'})

[soxofaan]

fixed the wrong kernel specs

and added the check as pre-commit hook

[soxofaan]

added github actions workflow now too