Streamline the quick start guide

[Kkkakania]

Summary

• replace the package-wide 76-cell tour with a focused RMSE workflow
• show how labeled dimensions align inputs and how dim controls reduction
• direct readers to the categorized API reference and tabular-data guide for deeper coverage

Closes #360.

Validation

• uvx pre-commit run --files docs/source/quick-start.ipynb
• executed the notebook from a clean uv run --extra complete environment with jupyter nbconvert --execute (all five code cells completed)

[review-notebook-app]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (e457985) to head (33dd162).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #460   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files          27       27           
  Lines        2841     2878   +37     
=======================================
+ Hits         2687     2722   +35     
- Misses        154      156    +2     

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