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/io/binary_serialization_api.rst

@@ -34,7 +34,9 @@ little-endian-encoded. All packets have a 4-byte header representing an
 integer, specifying the type of data.
 
 The lowest value two bytes are used to determine the type, while the highest value
-two bytes contain flags::
+two bytes contain flags:
+
+::
 
     base_type = val & 0xFFFF;
     flags = val >> 16;