Add caution message that this is outdated documentation

.gitignore

@@ -1,7 +1,5 @@
 _build/
 env/
-extensions/__pycache__/
-extensions/sphinx_tabs/
-extensions/*.pyc
+_extensions/__pycache__/
 *~
 .directory

index.rst

@@ -1,14 +1,15 @@
-.. Godot Engine documentation master file, created by
-   sphinx-quickstart on Sat Dec 12 16:57:47 2015.
-   You can adapt this file completely to your liking, but it should at least
-   contain the root `toctree` directive.
-
 Godot Docs – *2.1* branch
 =========================
 
-.. tip:: This is the documentation for the stable 2.1 branch.
-         Looking for the documentation of the current **3.0** branch?
-         `Have a look here <http://docs.godotengine.org/en/3.0>`_.
+.. caution::
+
+    This is the documentation for Godot 2.1, released in 2016. This version
+    is no longer supported, but its documentation is still available for
+    reference.
+
+    Please refer to the `stable <https://docs.godotengine.org/en/stable>`_
+    and `latest <https://docs.godotengine.org/en/latest>`_ branches of the
+    documentation for up-to-date information.
 
 Welcome to the official documentation of Godot Engine, the free and open source
 community-driven 2D and 3D game engine! If you are new to this documentation,
@@ -19,17 +20,6 @@ The table of contents below and in the sidebar should let you easily access the
 documentation for your topic of interest. You can also use the search function
 in the top left corner.
 
-.. note:: Godot Engine is an open source project developed by a community of
-          volunteers. It means that the documentation team can always use your
-          feedback and help to improve the tutorials and class reference. If
-          you do not manage to understand something, or cannot find what you
-          are looking for in the docs, help us make the documentation better
-          by letting us know!
-
-          Submit an issue or pull request on the `GitHub repository
-          <https://github.com/godotengine/godot-docs/issues>`_, or discuss with
-          us on the #godotengine-devel IRC channel!
-
 The main documentation for the site is organized into the following sections:
 
 .. toctree::
@@ -81,7 +71,7 @@ The main documentation for the site is organized into the following sections:
 
 .. Indices and tables
 .. ------------------
-.. 
+..
 .. * :ref:`genindex`
 .. * :ref:`modindex`
 .. * :ref:`search`