Integrate a custom device into Home Assistant using MQTT discovery so it appears automatically without manual YAML configuration

Steps

1. Ensure the MQTT integration is configured in Home Assistant and connected to your MQTT broker; MQTT discovery is enabled by default with the homeassistant prefix
2. From your device firmware or gateway, publish a discovery config message to homeassistant/<component>/<object_id>/config as a retained JSON payload; include at minimum: name, unique_id, state_topic, and device block with identifiers
3. Use abbreviated keys supported by Home Assistant (e.g., stat_t for state_topic, cmd_t for command_topic, uniq_id for unique_id) to keep payloads compact
4. Publish current device state to the state_topic; Home Assistant will subscribe automatically after processing the discovery message
5. For devices with multiple entities (sensors, switches, etc.), publish a separate discovery config for each entity under the same device block using shared identifiers so they are grouped under one device in the UI
6. To remove a device, publish an empty retained payload to its config topic; Home Assistant will remove the entity