doc: import Sphinx theme.css and pygments.css

We want to override the Sphinx theme only partially. So we need to
import the Sphinx theme.css. Cf.
https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html



We also need to import the pygments.css for syntax highlighing.

Use sans-serif for body.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>