doc: move and rename loaders section

PR-URL: nodejs/node#49261
Backport-PR-URL: nodejs/node#50669
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: sercher