Migrate official Notebooks from Gemini 2.0 to Gemini 3

[holtskinner]

This pull request migrates official notebooks and documentation to the Gemini 2.5/Gemini 3 model series, updates the project styleguide with modern best practices, and performs general maintenance across 40 files.

Key Changes

1. Model Migration & Optimization
   • Default Model Update: Updated MODEL_ID to gemini-2.5-flash or gemini-3-flash-preview across approximately 40 notebooks.
2. Styleguide Updates (.gemini/styleguide.md)
   • Deprecation: Formally deprecated Gemini 1.5 and 2.0 series in the styleguide for new samples.
3. General Maintenance
   • Smart Quote Removal: Replaced directional quotes (’, “, ”) with standard ASCII characters (', ") to prevent parsing issues in notebooks.
   • Spelling & Linting: Updated .github/actions/spelling/ configuration files (allow.txt, excludes.txt, line_forbidden.patterns) and fixed spelling errors repository-wide.
   • Formatting: Applied consistent formatting via nox -s format.

[koverholt]

Thanks for the updates! Confirming that the function calling notebooks are still relevant.

One note on the function calling notebooks (and probably all others being bumped to Gemini 3) is that using us-central1 as the LOCATION will not work. Rather, this will need to be set to global for all notebooks using Gemini 3.

With that change, the function calling notebooks all work, but I'll need to go through and update the function calling notebooks to match the narrative based on how the newest models behave. I can do that after this PR is merged.