kerneldoc: Add nicer CSS stylesheet for HTML docs

Import basic CSS stylesheet for the HTML documentation. The base for
the stylesheet is taken from:

	http://ds9a.nl/docbook/minimal-page.html



I customized the CSS a bit further, for example to add curvy corners
to example section and change the tint of gray. The HTML documentation
does not look that crude anymore.

Signed-off-by: Marek Vasut <marex@denx.de>