docs: stable-kernel-rules: remove code-labels tags and a indention level

Remove the 'code-block:: none' labels and switch to the shorter '::' to
reduce noise.

Remove a unneeded level of indentation, as that reduces the chance that
readers have to scroll sideways in some of the code blocks.

No text changes. Rendered html output looks like before, except for the
different level of indentation.

CC: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/755afbeafc8e1457154cb4b30ff4397f34326679.1714367921.git.linux@leemhuis.info