docs: improve statistical model search metadata

[loama]

What

• Adds focused search titles and descriptions to the GARCH and Holt Winters model notebooks.
• Adds focused search metadata to the GARCH and cross validation tutorials.

Why

The pages rank for high impression explanatory queries, but their current snippets do not clearly state the forecasting task, Python workflow, or evaluation content.

How

The new raw frontmatter cells describe the exact page purpose while leaving notebook content and execution unchanged.

Testing

• Parsed every changed notebook with jq.
• Converted all four notebooks through jupyter nbconvert to validate notebook structure.
• Ran git diff --check.

Checklist

• Existing routes remain unchanged.
• Metadata matches current StatsForecast behavior.
• No unsupported performance claims added.

[review-notebook-app]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 38 untouched benchmarks

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_{Comparing documentation/search-metadata (d29dfe9) with main (89e3b28)}