Added callbacks for node errors and fifo overflow

Add new doc system which should/might deploy to pages.

docs/examples.md

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+# Examples
+
+All C++ examples are built by default and registered as CTest smoke tests. Run them all with:
+
+```bash
+ctest --test-dir build -L examples
+```
+
+## Index
+
+| Example | What it shows |
+|---|---|
+| `01_hello_pipeline` | Linear pipeline, index-based port wiring |
+| `02_named_ports` | `in<>`/`out<>` name tags, named port access |
+| `03_multi_output` | Tuple-returning node, per-element routing |
+| `04_storage_policy` | `channel_storage_policy` specialisation |
+| `05_error_handling` | Diagnostics handler, overflow channel stats |
+| `06_watchdog` | Watchdog interval, stall detection |
+| `10_static_hello_pipeline` | `StaticNetwork` + `make_network()` |
+| `11_static_fanout` | `StaticNetwork` with `FanoutNode` |
+| `15_node_error_handler` | `set_error_handler()` — skip or stop on exception |
+| `16_event_callbacks` | `set_overflow_callback()`, `set_event_handler()` |
+
+## OpenCV examples (optional)
+
+Built only when OpenCV ≥ 4 is found:
+
+| Example | What it shows |
+|---|---|
+| `09_opencv_cellshade` | Real-time cell-shading on webcam; `MainThreadNode` for display |
+| `12_static_cellshade` | Same pipeline as a `StaticNetwork` |
+| `13_debug_cellshade` | Web debug UI overlay on the cell-shading pipeline |
+
+Run the cell-shading example:
+
+```bash
+./build/examples/09_opencv_cellshade
+# Press 'q' or close the window to stop.
+# Falls back to an animated synthetic pattern if no webcam is found.
+```