Replace 'code' directives with 'code-block' directives.

The `code` directive highlights all sources as GDScript. Other languages are
highlighted incorrectly at the moment, even if `.. code:: [language]` is
specified.

It does, however, work with the `code-block` directive. The reason seems to be
that this directive is Sphinx specific.

community/contributing/code_style_guidelines.rst

@@ -149,7 +149,7 @@ ones, the following rules should be followed:
 
 Example:
 
-.. code:: cpp
+.. code-block:: cpp
 
     /*************************************************************************/
     /*  my_new_file.h                                                        */
@@ -194,7 +194,7 @@ Example:
 
     #endif // MY_NEW_FILE_H
 
-.. code:: cpp
+.. code-block:: cpp
 
     /*************************************************************************/
     /*  my_new_file.cpp                                                      */

community/contributing/updating_the_class_reference.rst

@@ -182,7 +182,7 @@ Write in a clear and simple language. Always follow the :ref:`writing guidelines
 
 Here's how a class looks like in XML:
 
-.. code:: xml
+.. code-block:: xml
 
     <class name="Node2D" inherits="CanvasItem" category="Core">
         <brief_description>
@@ -255,7 +255,7 @@ Godot's class reference supports BBcode-like tags. They add nice formatting to t
 
 Use ``[codeblock]`` for pre-formatted code blocks. Inside ``[codeblock]``, always use **four spaces** for indentation (the parser will delete tabs). Example:
 
-.. code:: xml
+.. code-block:: xml
 
     [codeblock]
     func _ready():