Update Crusoe cluster docs (#3620)

* Changed "Crusoe Cloud" to "Crusoe" in the backend documentation for consistency.

* Update docs: feature Crusoe in cluster placement, rewrite cluster example

- Add Crusoe tab to fleets.md cluster placement backends
- Reorder backends.md to group Crusoe with other major cloud providers
- Rewrite examples/clusters/crusoe VMs section to use native backend
  instead of SSH fleets, with tested NCCL config

Made-with: Cursor

* Refactor documentation: Remove partners.md, update mkdocs.yml, and add Crusoe logo

* Linter fix

Author: peterschmidt85