Docs: public README, setup guide and architecture, generic deploy files, and the T3 Code notice

deploy/.env.example

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+# Copy to .env beside compose.yml. Without the APNS_ settings the gateway still runs, but it only
+# logs notifications instead of sending them.
+
+# The APNs auth key from your Apple developer account (Keys, with Apple Push Notifications on).
+APNS_KEY_ID=ABC123DEFG
+APNS_TEAM_ID=ABCDE12345
+# The .p8 file, placed in ./data so the container sees it at /data.
+APNS_KEY_PATH=/data/apns.p8
+# The watch app's bundle ID. Pushes are addressed to it.
+APNS_TOPIC=com.example.wristcode.watchkitapp
+
+TZ=UTC