Add some code snippets for the docs

docs/sphinx/rest_substitutions/snippets/python/converted/wx.ClientDC.1.py

@@ -0,0 +1,9 @@
+
+        # Create the initially empty label with the size big enough to show
+        # the given string.
+        dc = wx.ClientDC(self)
+        text = wx.StaticText(
+            self,
+            size=dc.GetTextExtent("String of max length"),
+            style=wx.ST_NO_AUTORESIZE
+           )

docs/sphinx/rest_substitutions/snippets/python/converted/wx.adv.GenericAnimationCtrl.1.py

@@ -0,0 +1 @@
+# Including headers is not required for Python.

docs/sphinx/rest_substitutions/snippets/python/converted/wx.grid.Grid.GetSelectedBlocks.1.py

@@ -0,0 +1,5 @@
+
+# For Python the returned GridBlocks object has a __iter__ method so iterating
+# in the Python way is possible.
+for block in self.grid.GetSelectedBlocks():
+    do_something(block)

docs/sphinx/rest_substitutions/snippets/python/converted/wx.grid.GridBlocks.1.py

@@ -0,0 +1,5 @@
+
+# For Python the returned GridBlocks object has a __iter__ method so iterating
+# in the Python way is possible.
+for block in self.grid.GetSelectedBlocks():
+    do_something(block)