Set Ruff E402 as default for notebooks

[redeboer]

Currently, the rule E402 (module-import-not-at-top-of-file) is deactivated for notebooks.

https://github.com/ComPWA/compwa.github.io/blob/bde4df9bf47c8e3165f4e3da62f7455913566d54/pyproject.toml#L256

We should activate this for notebooks and switch off that rule for the relevant notebooks through per-file-ignores.