docs: generic.css: font size tweaks

docs: generic.css: font size tweaks

- change font-family to just 'sans-serif' rather than hardcode a few
  font families. this means we abide the user's browser font setting,
  and makes us consistent with other sites like en.wikipedia.org
- raise font-size to 90%. this is what en.wikipedia.org uses.

With these two tweaks, libvirt.org text renders the same as
en.wikipedia.org with fedora firefox out of the box config. Previously
the font on libvirt.org was very small and difficult to read.