theme: don't use GNOME fonts

Oliver Smithrequested to merge
ollieparanoid/docs.postmarketos.org:remove-font into main

I've overlooked that the CSS file (which is adapted from GNOME's handbook as it says on top of the file) downloads fonts from static.gnome.org. Remove this part so they don't get unexpected requests to their servers when people read the postmarketOS docs.

The site almost looks the same without any extra font, so just remove it and have it load a bit faster.

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