docs(python/sedonadb): Ensure Python examples use sedona.db imports

[paleolimbot]

Our README and high-level Python documentation all use import sedona.db and sedona.db.connect(), however, all the Python docstrings predated that and used a different convention.

This PR adds a dummy sedona module and imports its to allow the docstrings to align with the documentation. This works because pytest --doctest-modules runs docstring examples in the context of the current module.