docs: point Android install at Discord for beta access + add community links

The Android app is in Google Play Internal Testing, so the public Play Store URL
doesn't resolve for non-testers. Lead the Android install instructions with a
"request a tester invite on Discord" CTA (the Play listing unlocks once a Google
account is added to the test track), and surface the Discord + r/Punktfunk
community links in the README, the docs intro, and the docs-site nav.

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

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

@@ -19,6 +19,8 @@ export function baseOptions(): BaseLayoutProps {
       { text: 'API', url: '/api' },
       { text: 'Website', url: 'https://punktfunk.unom.io' },
       { text: 'Source code', url: 'https://git.unom.io/unom/punktfunk' },
+      { text: 'Discord', url: 'https://discord.gg/kaPNvzMuGU' },
+      { text: 'Reddit', url: 'https://www.reddit.com/r/Punktfunk/' },
     ],
   }
 }