Replace uses of shorthand codeblock syntax to avoid issues with localization

Previously, many paragraphs in the documentation used `::` at the end of a line
as a replacement for a single colon (and `::` on the following line).
Due to this, the `::` was part of the extracted string for localization.
Weblate also showed a warning about the trailing `::`, as it's not
normally expected to be there.

This change requires translators to revalidate all modified strings.
However, considering many of those have broken code blocks due to translators
replacing `::` with `:`, it can be a good idea.

This also adds documentation writing guidelines for code blocks and admonitions.
This commit does not modify existing admonitions in any way (as this would
lead to a much larger diff, and require more localizable text to be revalidated
by translators).

tutorials/migrating/upgrading_to_godot_4.1.rst

@@ -281,7 +281,9 @@ There are two small changes:
 #. The first argument changes from ``const GDExtensionInterface *p_interface`` to ``GDExtensionInterfaceGetProcAddress p_get_proc_address``
 #. The constructor for the `init_obj` variable now receives ``p_get_proc_address`` as its first parameter
 
-You also need to add an extra ``compatibility_minimum`` line to your ``.gdextension`` file, so that it looks something like::
+You also need to add an extra ``compatibility_minimum`` line to your ``.gdextension`` file, so that it looks something like:
+
+::
 
   [configuration]