Add a small Start here section to the root gallery

## Goal
Help first-time visitors know which experiment to open first without turning the root page into a large marketing landing page.

## Scope
Add a compact editorial **Start here** section to the root `index.qmd`, above the full app catalog.

Suggested initial picks:
- **K-means Step by Step** — learn how an iterative clustering algorithm converges;
- **Underfitting and Overfitting** — see the generalization trade-off visually;
- **ROC Curve** — explore thresholds, discrimination and classification metrics.

## Important distinction
This should be a **section**, not a new tag/category. The existing category filter should remain subject-matter based; `Start here` is editorial guidance, not taxonomy.

## Design direction
- Keep it small: approximately three featured cards/links.
- Reuse the current visual language where possible.
- Avoid adding long explanatory copy, difficulty systems, course-like navigation or a new information architecture.
- The regular catalog remains the primary body of the page immediately below.
- Prefer explicit hand-picked links over adding more metadata machinery to every `DESCRIPTION` file unless that becomes clearly useful later.

## Done when
A visitor unfamiliar with the project sees 2–3 obvious entry points before the complete catalog, while the root page still feels lightweight and data/experiment-first.

Start in the root index.qmd and inspect how the existing app catalog presents links or cards. Add a compact Start here section with explicit links to the three suggested experiments, keeping the catalog directly below it and reusing the current visual language. Done means unfamiliar visitors see 2–3 entry points without a new category, metadata system, or lengthy navigation.

Written by the indexing model from the issue text.