feat: Display index and multi-index in anywidget mode

[shuoweil]

This PR introduces several improvements related to the anywidget display mode. For displaying multi-index DataFrames in anywidget mode, it will be rendered with a nested "semi-exploding" view, similar to BigQuery web UI.

Added Index and Multi-Index DataFrame Example to Notebook:

• The notebooks/dataframes/anywidget_mode.ipynb notebook now includes a new section demonstrating how to display a DataFrame with a MultiIndex using real data from the PyPI public dataset. This enhances the notebook's examples for complex data structures.

Added Index and Multi-Index System Tests:

• The tests were made more robust by replacing brittle, generic string assertions with precise checks for unique data values within rendered HTML table cells.

See an example here: screen/6nyCvGjzpRwM2nW

Fixes #<459515995> 🦕

[review-notebook-app[bot]]

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