doc: Set default language for syntax highlighting to "none"

Force code blocks with no 'language' explicitly set to render with no
highlighting. The current default was to use python (bad), and there
would also be an option to set it to "guess" but this doesn't work all
so well (ex. some C blocks would be detected as Transact-SQL!).

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>