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>

README.md

@@ -8,6 +8,9 @@ or games — each device at its **own native resolution and refresh rate**, over
 [How It Works](https://docs.punktfunk.unom.io/docs/how-it-works) or the
 [Quick Start](https://docs.punktfunk.unom.io/docs/quickstart).
 
+💬 **Community: [Discord](https://discord.gg/kaPNvzMuGU)** — chat, support, and **Android beta
+access** · **[r/Punktfunk](https://www.reddit.com/r/Punktfunk/)**.
+
 punktfunk pairs a **virtual-display streaming host** with native clients on every platform. It speaks
 the existing **GameStream** protocol, so any [Moonlight](https://moonlight-stream.org/) client works
 day one — and adds its own faster **`punktfunk/1`** protocol that breaks the ~1 Gbps FEC wall with a