Move examples in docs to notebooks

[FrancescMartiEscofetQC]

When we add the github action to build the docs we need to remember to add the step:

      - name: Check notebooks mypy # TODO: Make sure that the rest of pchs also pass
        run: |
          jupyter nbconvert --to script docs/examples/*.ipynb
          for file in docs/examples/*.txt; do mv -- "$file" "${file%.txt}.py"; done
          mypy docs/examples/*.py

Checklist

• Added a CHANGELOG.rst entry