feat(docs): add RSS 2.0 feed for documentation site

[Copilot]

Implements RSS feed at /rss.xml for documentation updates.

Changes

• RSS endpoint (docs/app/rss.xml/route.ts): Next.js route handler generating RSS 2.0 XML with all documentation pages from Fumadocs source
• Autodiscovery link (docs/app/layout.tsx): Added <link rel="alternate"> to HTML head for RSS reader detection

Implementation Details

• XML escaping for text content (titles, descriptions); URLs kept unescaped per RSS spec
• Date validation prevents invalid XML from malformed lastModified fields
• ISR with 3600s revalidation matching cache headers
• Type-safe page data handling with explicit interface

// URLs remain valid, text content is escaped
const title = escapeXml(page.data.title || 'Untitled');
const link = `${baseUrl}${page.url}`; // Not escaped

Feed accessible at https://docs.copilotkit.ai/rss.xml in production.

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