Add `examples/arena_hard.py` and remove from `distilabel` core

[alvarobartt]

Description

As recently discussed internally, maintaining the benchmarks may be too time consuming and hard, since there are a lot of things going on and benchmarks tend to be maintained separately. So on, we internally decided to remove the implementation from the distilabel core for the moment, and provide that as an example instead, that anyone can use as reference for building benchmarks with distilabel.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #741 will not alter performance

_{Comparing remove-benchmarks (b1ac3c7) with develop (356a4a3)}

Summary

✅ 1 untouched benchmarks