doc: activate `no-multiple-empty-lines` rule

This enables the `no-multiple-empty-lines` eslint rule for the docs.

PR-URL: https://github.com/nodejs/node/pull/18747


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>