doc: document JavaScript tool for benchmark comparison

This makes it simpler for everyone to do the statistical analysis since
it doesn't require to install R and its dependencies.

Refs: https://github.com/targos/node-benchmark-compare

PR-URL: https://github.com/nodejs/node/pull/39835


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>