doc: make version picker usable on mobile

This makes the version picker usable on mobile devices.
Previously, the version picker was difficult to select from. This
change makes the #gtoc > ul > li elements have a display
of `block` instead of `inline`.

PR-URL: https://github.com/nodejs/node/pull/39958


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>