Add documentation for creating new benchmark tests

[mrickard]

Description

Since we've edited benchmark tests and the benchmark testing framework to be able to be run in the current codebase, we have an opportunity to add tests to uncovered parts of the agent. As such, the process of creating benchmark tests should be documented.

Additional context

There's a high-level overview in Google Docs. Otherwise, not much is documented.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-337788