docs: clarify Rstest testing framework positioning

[9aoy]

Summary

• Position Rstest clearly as a fast, modern JavaScript testing framework across the homepage, SEO metadata, README, and introduction docs.
• Present Rspack as the underlying engine and an additional integration advantage instead of the product category.
• Reorder homepage features to lead with familiar testing APIs, modern defaults, and performance.

Related Links

None

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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Rsdoctor Bundle Diff Analysis

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Found 13 projects in monorepo, 0 projects with changes.

📊 Quick Summary

Project	Total Size	Change
adapter-rsbuild	6.1 KB	0
adapter-rslib	26.3 KB	0
adapter-rspack	9.2 KB	0
browser-react	6.4 KB	0
browser-ui	810.3 KB	0
coverage-v8	80.0 KB	0
coverage-istanbul	18.8 KB	0
browser	2.1 MB	0
playwright	50.0 KB	0
core/main	1.9 MB	0
core/browser	938.8 KB	0
vscode/extension	47.8 MB	0
vscode/worker	14.8 KB	0

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