doc: reduce font size on smaller screens

The font size for tablet sized screens (or half a laptop screen) is
overly large, reduce it to a more readable size.

Fixes: https://github.com/nodejs/nodejs.org/issues/919

PR-URL: https://github.com/nodejs/node/pull/11695
Fixes: https://github.com/nodejs/nodejs.org#919


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>