Add documentation and class reference articles.

documentation/overview.rst

@@ -0,0 +1,19 @@
+Overview
+========
+
+There are two separate resources referred to as "documentation" in Godot:
+
+- **The class reference.** This is the documentation for the complete Godot API
+  as exposed to GDScript and the other scripting languages. It can be consulted
+  offline, directly in Godot's code editor, or online at Godot :ref:`Class Reference
+  <doc_class_reference>`. To contribute to the class reference, you have to edit the
+  XML file corresponding to the class and make a pull request.
+  See :ref:`doc_updating_the_class_reference` and :ref:`doc_class_reference_primer`
+  for more details.
+
+- **The tutorials and engine documentation and its translations.**
+  This is the part you are reading now, which is distributed in the HTML format.
+  Its contents are generated from plain text files in the reStructured Text
+  (rst) format, to which you can contribute via pull requests on the
+  `godot-docs <https://github.com/godotengine/godot-docs>`_ GitHub repository.
+  See :ref:`doc_contributing_to_the_documentation` for more details.