doc,benchmark: move benchmark guide to benchmark directory

The doc/guides directory looks like a place for guides to using Node.js
but it's actually where we store development/internal documentation.
Move the benchmark guide to the benchmark directory where someone (who
doesn't know that bit of somewhat-surprising information) is more likely
to find it.

PR-URL: https://github.com/nodejs/node/pull/30781


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>