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/export/exporting_projects.rst

@@ -248,7 +248,9 @@ depending on your needs.
     Due to a `known bug <https://github.com/godotengine/godot/pull/42123>`__,
     when using a ZIP file as a pack file, the exported binary will not try to use
     it automatically. Therefore, you have to create a *launcher script* that
-    the player can double-click or run from a terminal to launch the project::
+    the player can double-click or run from a terminal to launch the project:
+
+    ::
 
         :: launch.bat (Windows)
         @echo off