Add a simple tooltip to the sidebar (#6295)
## Summary
Not perfect, but IMO helpful:
<img width="1792" alt="Screen Shot 2023-08-02 at 9 29 24 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/e613e918-75cb-475e-9ea4-f833d1a0b5f6">
<img width="1792" alt="Screen Shot 2023-08-02 at 9 29 20 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/bb3efdfe-40e1-45b5-b774-082521b2d214">
