[Perf] Create documentation for writing and running perf tests

Create language-agnostic documentation for writing and running perf tests.

### Test Lifecycle
1. Create parallel instances of the test class
2. Call `globalSetupAsync()` on the first instance
3. Call `setupAsync()` on all instances (in parallel)
4. For `warmup` seconds, call `run()/runAsync()` in a loop
5. For `duration` seconds, call `run()/runAsync()` in a loop
6. Call `cleanupAsync()` on all instances (in parallel)
7. Call `globalCleanupAsync()` on the first instance

### Tips
1. You need to think carefully about what is shared vs unique across parallel instances
2. Always test changes with `parallel > 1`. If you only test with `parallel == 1` you can miss issues.