Give specific instructions on writing UI extensions

* This goes through a worked example of a simple extension.

docs/python_api/ui_extension_tutorial/extension.json

@@ -0,0 +1,9 @@
+{
+	"extension_api": 1,
+	"name": "Tutorial extension",
+	"version": "1.0",
+	"minimum_renderdoc": "1.12",
+	"description": "Tutorial extension from the documentation.",
+	"author": "Baldur Karlsson <baldurk@baldurk.org>",
+	"url": "https://github.com/baldurk/renderdoc"
+}