Output only satellite - part 2 (#278)

Introduce a new `--output-only` command-line flag and corresponding `ENABLE_OUTPUT_ONLY` environment variable. This allows users to run the assistant in output-only mode, where it only supports announcements. The change includes updates to configuration examples, documentation, and the Docker entrypoint script.

.env.example

@@ -67,3 +67,6 @@ LVA_PULSE_COOKIE="/run/user/${LVA_USER_ID}/pulse/cookie"
 # PROCESSING_SOUND="sounds/processing.wav"
 # MUTE_SOUND="sounds/mute_switch_on.flac"
 # UNMUTE_SOUND="sounds/mute_switch_off.flac"
+
+### Enable output-only mode (optional):
+# ENABLE_OUTPUT_ONLY="1"