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/package.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<package format="1" xmlns="https://wiki.freecad.org/Package_Metadata">
+  <name>example-addon</name>
+  <description>Example addon template for Kindred Create</description>
+  <version>0.1.0</version>
+  <maintainer email="you@example.com">Your Name</maintainer>
+  <license file="LICENSE">LGPL-2.1-or-later</license>
+  <url type="repository">https://example.com/your-addon</url>
+
+  <content>
+    <!-- A <workbench> tag is required for InitGui.py to be loaded,
+         even if the addon does not register an actual workbench. -->
+    <workbench>
+      <classname>ExampleAddonProvider</classname>
+      <subdirectory>example_addon</subdirectory>
+    </workbench>
+  </content>
+
+  <!-- Kindred-specific extensions — ignored by stock FreeCAD. -->
+  <kindred>
+    <min_create_version>0.1.0</min_create_version>
+    <load_priority>80</load_priority>
+    <dependencies>
+      <dependency>sdk</dependency>
+    </dependencies>
+  </kindred>
+</package>