C#: Add documentation for diagnostic GD0003

Author: raulsntos

tutorials/scripting/c_sharp/diagnostics/GD0003.rst

@@ -0,0 +1,49 @@
+GD0003: Found multiple classes with the same name in the same script file
+=========================================================================
+
+====================================  ======================================
+                                      Value
+====================================  ======================================
+**Rule ID**                           GD0003
+**Category**                          Usage
+**Fix is breaking or non-breaking**   Non-breaking
+**Enabled by default**                Yes
+====================================  ======================================
+
+Cause
+-----
+
+A script file contains multiple types that derives from ``GodotObject`` with
+a name that matches the script file. Only one type in the script file should
+match the file name.
+
+Rule description
+----------------
+
+Godot requires scripts to have a unique path so every type must be defined on its
+own file and the type name must match the file name.
+
+.. code-block:: csharp
+
+    public partial class MyNode : Node { }
+
+    namespace DifferentNamespace
+    {
+        // Invalid because there's already a type with the name MyNode in this file.
+        public partial class MyNode : Node { }
+    }
+
+    // Invalid because there's already a type with the name MyNode in this file.
+    public partial class MyNode<T> : Node { }
+
+How to fix violations
+---------------------
+
+To fix a violation of this rule, move each type declaration to a different file.
+
+When to suppress warnings
+-------------------------
+
+Do not suppress a warning from this rule. Types that derive from ``GodotObject``
+must have a unique path otherwise the engine can't load the script by path,
+resulting in unexpected runtime errors.

tutorials/scripting/c_sharp/diagnostics/index.rst

@@ -17,6 +17,7 @@ Rules
 
    GD0001
    GD0002
+   GD0003
    GD0101
    GD0102
    GD0103