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/ui/bbcode_in_richtextlabel.rst

@@ -581,7 +581,9 @@ To handle clicked ``[url]`` tags, connect the ``RichTextLabel`` node's
 :ref:`meta_clicked <class_RichTextLabel_signal_meta_clicked>` signal to a script function.
 
 For example, the following method can be connected to ``meta_clicked`` to open
-clicked URLs using the user's default web browser::
+clicked URLs using the user's default web browser:
+
+::
 
     # This assumes RichTextLabel's `meta_clicked` signal was connected to
     # the function below using the signal connection dialog.