feat(docs-site): add a Gitea repo link to the nav

Link to https://git.unom.io/unom/punktfunk in the top bar, alongside the
Docs and Website links.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/src/lib/layout.shared.tsx

@@ -17,6 +17,7 @@ export function baseOptions(): BaseLayoutProps {
     links: [
       { text: 'Docs', url: '/docs' },
       { text: 'Website', url: 'https://punktfunk.unom.io' },
+      { text: 'Source code', url: 'https://git.unom.io/unom/punktfunk' },
     ],
   }
 }