doc: eliminate dead space in API section's sidebar

This commit improves overall usability on desktop by removing the "dead"
(non-interactive) regions between links in the API section's navigation.
No visible changes have been made; this enhancement is purely tactile in
nature.

PR-URL: https://github.com/nodejs/node/pull/33469


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>