DOC: use constants in performance-comparisons.ipynb (#15215)

I've simplified the performance comparisons notebook by setting constants which can be adjusted at the top of each section e.g. `num_rows`. This makes it easier for anyone running this to adjust the value and hopefully not encounter memory values. It can also help with testing these benchmarks on dataframes of various lengths. I've stripped the output as I was working on a A10G and I couldn't run with the current `num_rows` value. I also didn't want to commit the results which may differ compared to the H100 which is used currently and I would rather the results be committed by the RAPIDS team. I can confirm the notebook runs end-to-end (you can see my version here: https://github.com/raybellwaves/cudf-performance-comparisons/blob/main/performance-comparisons.ipynb with smaller `num_rows` and smaller `timeit_number` on a A10G (EC2 machine))

Authors:
  - Ray Bell (https://github.com/raybellwaves)
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)

URL: #15215