[codex] Add GTFS MQTT connector and connector docs

[jillesvangurp]

Summary

• add an MQTT variant of the GTFS connector that reuses the existing GTFS mapping and REST bootstrap flow
• add MQTT-specific connector helper/config support plus updated GTFS setup docs and env examples
• add connector overview/WebSocket/MQTT docs and point the MQTT guide at the concrete GTFS example

Why

The repo previously documented MQTT connector development without a bundled MQTT example. This change adds a real GTFS MQTT connector so the MQTT guide can point at an implemented example instead of a documentation-only path.

Validation

• just bootstrap
• just generate
• just check
• uv sync --project connectors/gtfs
• smoke test: uv run --project connectors/gtfs python connectors/gtfs/connector_mqtt.py --env-file /tmp/gtfs-mqtt.env --once
• live run against the local demo stack for 190 seconds
  • 13 polls with data
  • 108 MQTT Location publishes total
  • per-poll counts: 9,9,9,9,8,8,8,8,8,8,8,8,8
  • downstream WebSocket observer saw 107 location_updates payload items plus one startup/subscription artifact