docs: add integration_duckdb_example and refactor names of prev notebooks

[federicsp]

Rationale for this change

This PR adds a new example notebook demonstrating integration between PyIceberg and DuckDB, including:

• Creating a temporary warehouse and SQL catalog
• Creating Iceberg tables and appending data (first and second snapshots)
• Comparing snapshots in DuckDB (added/removed rows)
• Adding computed columns, filtering, and aggregating data

It also refactors and renames previous notebooks for clarity and consistency.

Are these changes tested?

Yes, the notebooks have been tested.

Are there any user-facing changes?

No, these are documentation/example changes only.

[Fokko]

Thanks @federicsp for adding this! This will help folks up to speed with DuckDB and PyIceberg.

Regarding testing, what do you think of adding someting like Papermill: https://papermill.readthedocs.io/en/latest/. This will allow us testing the Notebook in the CI. This ensures that the notebooks keep working.

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