doc: move and rename loaders section

PR-URL: nodejs/node#49261
Backport-PR-URL: nodejs/node#50669
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Jacob Smith <[email protected]>
Reviewed-By: Rafael Gonzaga <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>

Author: sercher