Update telemetry docs from UDP to MQTT

[TalkingJupiter]

Context

The firmware has moved toward MQTT, but repo docs still describe UDP telemetry and UDP listener validation.

Files to update

• README.md
• Connecting_ESP32_Telemetry_to_Radxa.md
• Review host/mosquitto_docker_setup.md for consistency with firmware topics.

Required doc changes

• Replace UDP telemetry transport language with MQTT over W5500 Ethernet.
• Document broker host/port from HostConfig.h.
• Document topics:
  • repacss/devices/<mac>/hello
  • repacss/devices/<mac>/telemetry
  • repacss/devices/<mac>/config
• Replace UDP listener examples with mosquitto_sub examples.
• Keep the A/B heartbeat failover explanation.

Acceptance criteria

• Docs no longer instruct users to use nc -klu or RADXA_UDP_PORT for current firmware.
• Docs show how to verify MQTT telemetry with Mosquitto.
• Docs explain the expected one-active-sender behavior for Controller A/B.