docs: Use the geistdocs Turborepo logo in the navbar (#13682)
Follow-up to #13681 (simplify and correct logo symbol)
| **Before** | **After** |
| ------------- | ------------- |
| <img width="772" height="534" alt="CleanShot 2026-08-04 at 22 22
22@2x"
src="https://github.com/user-attachments/assets/0dbc4e2d-d730-4fc4-8dab-a59717221107"
/> | <img width="736" height="478" alt="CleanShot 2026-08-04 at 22 21
39@2x"
src="https://github.com/user-attachments/assets/2b8e41d4-acb6-404c-bdf1-29337b05d3d9"
/> |
