Add marwoj.net MQTT broker preset 🤖🤖

[MarekWo]

Summary

Adds a built-in MQTT preset for the Polish MeshCore community broker at
mqtt.marwoj.net.

The broker uses MQTT over TLS on port 8883 with username/password
authentication and the standard MeshCore topic layout, so users can
configure it with a single command:

set mqttN.preset marwoj

Configuration

• Endpoint: mqtts://mqtt.marwoj.net:8883
• Authentication: username/password embedded in the preset, following the
  existing tennmesh / nashmesh / ctmesh pattern
• TLS CA: the existing ISRG_ROOT_X1 constant (the server presents a valid
  Let's Encrypt chain, so no new CA certificate is added)
• Topic style: MQTT_TOPIC_MESHCORE, retain enabled, keepalive 55

The credentials are a dedicated public firmware account created for this
preset; they are intentionally not a private or admin credential.

Changes

• src/helpers/MQTTPresets.h — add the marwoj entry and bump
  MQTT_PRESET_COUNT from 36 to 37
• MQTT_IMPLEMENTATION.md — add the broker to the presets table and to the
  two notes listing presets that ship fixed credentials in firmware

Testing

• Endpoint and credentials tested on a physical MeshCore Observer device
  using the equivalent custom slot configuration: the slot came up as
  connected over TLS, authenticated successfully, and published without
  publish errors.
• TLS chain verified independently with openssl s_client -verify_return_error
  (verify return code 0).
• pio run -e heltec_v4_repeater_observer_mqtt builds clean with this change.
• The native gtest suite could not be run locally (no host compiler on this
  machine); the preset table was instead compile-checked with static asserts
  confirming MQTT_PRESET_COUNT matches the number of initialisers.