bitdynamics-ab/canton-devkit: docs/troubleshooting.md

# Troubleshooting Failure modes and fixes. Start with `canton-devkit localnet doctor` — it runs the same host preflight as `localnet up` (Docker CLI, daemon, Compose v2, disk + memory headroom, platform, port availability; pass `--version <tag>` to use that version's memory thresholds) and prints ta