update benchmark docs to mention divan

[CrockAgile]

A contributor was unclear while working on #154 if divan or criterion was the main benchmark suite (link).

Documentation should call out the current state of the Divan benchmarks, and decisions such as:

• when to use Divan vs Criterion
• whether both should be kept
• if both are kept, how to run only one of them (cargo bench by default will run both)

[Carlyle-Foster]

i'm not very familiar with their differences, i get the impression that divan is more typesafe? the black_box feature also seems useful but maybe criterion does that kind of thing under the hood?

[Carlyle-Foster]

i definitely don't think they're different enough to justify keeping both and duplicating all the benches, which btw are already out of sync. better to flip a coin than keep both