feat: create examples and corresponding build config

examples/relay/README.md

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+# Relay Service Example
+
+A minimal example that uses the framework's `NodeContext`, `NetworkManager`, `TaskManager`, and `ApiServer` to control a relay via REST and log status periodically as a task.
+
+- Default relay pin: `GPIO0` (ESP-01). Override with `-DRELAY_PIN=<pin>`.
+- WiFi and API port are configured in `src/Config.cpp`.
+
+## Build & Upload
+
+- ESP‑01S:
+```bash
+pio run -e esp01_1m_relay -t upload
+```
+- D1 Mini:
+```bash
+pio run -e d1_mini_relay -t upload
+```
+
+Monitor serial logs:
+```bash
+pio device monitor -b 115200
+```
+
+Assume the device IP is 192.168.1.50 below (replace with your device's IP shown in serial output).
+
+## Relay API
+
+- Get relay status
+```bash
+curl http://192.168.1.50/api/relay/status
+```
+
+- Turn relay ON
+```bash
+curl -X POST http://192.168.1.50/api/relay/set -d state=on
+```
+
+- Turn relay OFF
+```bash
+curl -X POST http://192.168.1.50/api/relay/set -d state=off
+```
+
+- Toggle relay
+```bash
+curl -X POST http://192.168.1.50/api/relay/set -d state=toggle
+```
+
+Notes:
+- Requests use `application/x-www-form-urlencoded` by default when using `curl -d`.
+
+## Task Management (optional)
+The example registers a periodic task `relay_status_print` that logs the current relay state.
+
+- Fetch all task statuses
+```bash
+curl http://192.168.1.50/api/tasks/status
+```
+
+- Query a specific task status
+```bash
+curl -X POST http://192.168.1.50/api/tasks/control \
+  -d task=relay_status_print -d action=status
+```
+
+- Enable / Disable the task
+```bash
+curl -X POST http://192.168.1.50/api/tasks/control \
+  -d task=relay_status_print -d action=enable
+
+curl -X POST http://192.168.1.50/api/tasks/control \
+  -d task=relay_status_print -d action=disable
+```
+
+## General System Endpoints (optional)
+- Node status
+```bash
+curl http://192.168.1.50/api/node/status
+```
+
+- Restart device
+```bash
+curl -X POST http://192.168.1.50/api/node/restart
+```