Add a Godot 3 to Godot 4 upgrade guide (#6393)

Author: Calinou

about/release_policy.rst

@@ -88,7 +88,7 @@ on GitHub.
 +-------------+----------------------+--------------------------------------------------------------------------+
 | **Version** | **Release date**     | **Support level**                                                        |
 +-------------+----------------------+--------------------------------------------------------------------------+
-| Godot 4.0   | Q4 2022              | |unstable| *Alpha.* Current focus of development (unstable).             |
+| Godot 4.0   | Q4 2022              | |unstable| *Beta.* Current focus of development (unstable).              |
 +-------------+----------------------+--------------------------------------------------------------------------+
 | Godot 3.6   | Q4 2022              | |supported| *Beta.* Receives new features as well as bug fixes while     |
 |             |                      | under development.                                                       |
@@ -130,6 +130,10 @@ on GitHub.
 Pre-release Godot versions aren't intended to be used in production and are
 provided for testing purposes only.
 
+.. seealso::
+
+    See :ref:`doc_upgrading_to_godot_4` for instructions on migrating a project.
+
 .. _doc_release_policy_when_is_next_release_out:
 
 When is the next release out?

tutorials/editor/index.rst

@@ -61,3 +61,12 @@ a project that's only 2D or only 3D and don't want to see what you don't need.
    :name: toc-editor-features
 
    managing_editor_features
+
+Upgrading to Godot 4
+--------------------
+
+.. toctree::
+   :maxdepth: 1
+   :name: toc-editor-upgrading
+
+   upgrading_to_godot_4