docs: add RSS link to homepage

[chenjiahan]

Summary

Add the RSS link to homepage.

Related Links

web-infra-dev/rspress#828

Checklist

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

[netlify]

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Lighthouse	1 paths audited Performance: 75 (🟢 up 1 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 92 (no change from production) PWA: - View the detailed breakdown and full score reports

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