doc: add padding in the sidebar column

PR-URL: https://github.com/nodejs/node/pull/34665


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Evan Lucas <evanlucas@me.com>