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 cus

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>

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