Include more performance numbers

## Communication goals (e.g. detailed howto vs orientation)

We have [benchmarks](http://docs.citusdata.com/en/v7.5/performance/performance_tuning.html#insert-throughput) for inserts and [instructions](http://docs.citusdata.com/en/v7.5/extra/write_throughput_benchmark.html) to replicate the numbers, but we should include benchmarks for other things too.

* \copy speed
* TODO: learn what else we want to cover

### Good locations for content in docs structure

New top-level Performance section

### Are there relevant blog posts or outside documentation about the concept/feature?

Start with the existing insert benchmarks at docs.citusdata.com/en/v7.5/performance/performance_tuning.html#insert-throughput and the replication instructions at docs.citusdata.com/en/v7.5/extra/write_throughput_benchmark.html. Determine which additional performance topics belong in a new top-level Performance section, including \copy speed, and document reproducible benchmarks with clear scope and results.

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