docs-rst: Use better colors for note/warning/attention boxes

Instead of painting the box with gray, let's use a colored
box. IMHO, that makes easier to warn users about some issue
pointed by the Sphinx. It also matches to what we do already
with the HTML output.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>