Tweak hierarchy of benchmark docs (#5720)
## Summary
Before:
<img width="309" alt="Screen Shot 2023-07-12 at 4 33 23 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/b4a29dc5-183d-479f-8028-f47157b87e0e">
After:
<img width="281" alt="Screen Shot 2023-07-12 at 4 33 32 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/316859d3-db90-4595-8c07-b4bb6543ac4d">
