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).
