reorganize notebooks and example data

[LOCEANlloydizard]

The example notebooks currently live inside the documentation source tree and rely on external Google Drive links for example data. We could simplify the repository structure and improve reproducibility by moving the notebooks to a dedicated location and using GitHub Release assets for the example data!

Tasks

• Move the example notebooks to a top-level notebooks/ directory
• Replace Google Drive downloads with GitHub Release assets (after Host example data as GitHub Release assets #253)
• Update the notebooks to download/cache the example data automatically
• Add generated images and processed/example data to .gitignore
• Update the examples to demonstrate and test the complete workflow:
  • open a raw file
  • save as a converted Zarr
  • reopen the converted Zarr
  • compute Sv/MVBS
  • save/read an MVBS Zarr
  • render an echogram with Echoshader
• Verify that the notebooks still build correctly on Read the Docs
• Verify that all notebooks run successfully from a clean environment using the current pyproject.toml installation workflow