DOC: Expand data table representation with labels and index (#62358)

[akhilw0811]

• Replaces the intro tutorial diagram with an SVG that clearly shows column labels (header) and the row index.
• Adds a short note explaining the role of the index and links to :ref:indexing.
• Verified locally with python -m sphinx -b html source _build/html.

Closes #62358

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this.

[mroeschke]

Thanks for this pull request, but we already have another contributor working on this in #62466 so closing for now to give that contributor a chance to finish.