Add benchmark to CI test and document CI.

This invokes the benchmarks tool to do simple performance tests on the commit under test.

At the moment, the performance test can only be run on specific hardware, to keep performance tests as repeatable as possible. We only have one runner at the moment which can run them, so this is a single point of failure. However, the performance test isn't required to pass; it will only show as a warning if the performance test can't be run, or fails.