docs: Include example dataset files in repo

[wtn]

Fixes #10186.

Rust examples fail with No such file or directory because dataset files (parquet, ipc, ndjson) are gitignored and only generated on-demand by init_files() during test runs.

Fix

Add pre-generated dataset files directly to the repository so examples work out-of-the-box.

• Add foods1/foods2/null_nutriscore in parquet, ipc, ndjson formats
• Remove examples/datasets/.gitignore that blocked these files
• Add negation patterns to root .gitignore for examples/datasets/

[juansolm]

@wtn could we make this generic so we can add more datasets to the resository?

See this issue with regard to scattered datasets: #23763

I would propose we rename examples to datasets and work with dataset folders inside to organize
So your example would be datasets/food/<files/subfolders>

[wtn]

@juansolm Would you like to collaborate on this PR?

If so, I can add you to my fork.

[juansolm]

I'm unable right now, thanks for the offer.