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/content/docs/clients.md

@@ -64,8 +64,10 @@ DualSense feedback, automatic host discovery, PIN pairing with pinned reconnects
 overlay — with D-pad and game-controller focus navigation for the couch. It builds from the
 `clients/android` directory (Kotlin + a shared Rust core).
 
-Install it from **Google Play** — see [Install a Client](/docs/install-client#android). Open the app,
-pick your host, [pair](/docs/pairing) once, and stream.
+The app is in **Google Play Internal Testing** — request a tester invite on our
+[**Discord**](https://discord.gg/kaPNvzMuGU) and we'll add you (see
+[Install a Client](/docs/install-client#android)). Once added, open the app, pick your host,
+[pair](/docs/pairing) once, and stream.
 
 ## Windows desktop client