Add RSS icon

[HiDeoo]

What kind of changes does this PR include?

• Changes or translations of Starlight docs site content
• Changes to Starlight code

Description

As requested on Discord, this pull request adds an RSS feed icon and includes it in the social icons options.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3a03a17

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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[netlify]

✅ Deploy Preview for astro-starlight ready!

Name	Link
🔨 Latest commit	3a03a17
🔍 Latest deploy log	https://app.netlify.com/sites/astro-starlight/deploys/651ab89e4c9c440008d7a14e
😎 Deploy Preview	https://deploy-preview-792--astro-starlight.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 100 (no change from production) Accessibility: 100 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: - View the detailed breakdown and full score reports

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[delucis]

Thanks @HiDeoo — LGTM!