add example code

Author: hx2A

py5_docs/Reference/api_en/Py5Graphics_is3d.txt

@@ -11,3 +11,19 @@ is3d() -> bool
 Boolean value reflecting if the Py5Graphics object is or is not a 3D renderer.
 
 This will be `True` if the renderer is a 3D renderer such as `P3D`.
+
+@@ example
+def setup():
+    py5.size(200, 200)
+    g = py5.get_graphics()
+
+    # prints False
+    print(g.is3d())
+
+@@ example
+def setup():
+    py5.size(200, 200, py5.P3D)
+    g = py5.get_graphics()
+
+    # prints True
+    print(g.is3d())