docs: add example addon template (#395)

- Create docs/examples/example-addon/ with a complete, copy-paste-ready
  addon skeleton demonstrating command registration, context injection,
  dock panels, lifecycle hooks, and event bus subscription
- Add Examples section to docs/src/SUMMARY.md
- Add quick-start cross-reference in writing-an-addon.md

docs/examples/example-addon/example_addon/panel.py

@@ -0,0 +1,21 @@
+"""Example addon dock panel.
+
+The factory function is passed to ``kindred_sdk.register_dock_panel()``
+and called once after the configured delay to create the widget.
+"""
+
+
+def create_panel():
+    """Create the example dock panel widget.
+
+    Must return a QWidget. The widget is embedded in a QDockWidget and
+    managed by FreeCAD's DockWindowManager.
+    """
+    from PySide6 import QtWidgets
+
+    widget = QtWidgets.QWidget()
+    layout = QtWidgets.QVBoxLayout(widget)
+    layout.addWidget(QtWidgets.QLabel("Example Addon Panel"))
+    layout.addWidget(QtWidgets.QLabel("Replace this with your content."))
+    layout.addStretch()
+    return widget