add synchronous benchmarks

[ArtemIsmagilov]

Please add synchronous libraries so we can compare them with blocking calls. Ideally, add Celery Dramatiq RQ.

[Graeme22]

As the docs mention, sync tasks are second-class citizens, so I'm not sure it makes sense to benchmark against dedicated sync task queues. If you want to modify the benchmark code yourself and post some results I'm sure some people will find it helpful, but I don't think it belongs in the official docs.

[Graeme22]

Sync tasks are really just supported to help people who are migrating large codebases to async and haven't finished yet.