Document and test basic rollout use cases

[weyfonk]

The goal here is to be able to answer the following questions, with tests and documentation, about basic aspects of rollout:

• [ ] How does auto-partitioning work? Does e.g. 10% of 4 clusters result in 1 partition?
• [ ] Example use case from https://github.com/rancher/fleet/issues/2891
• [ ] rollout.go: does manual partitioning really distribute the remaining partitions as stated in the docs?

Documentation must include user-facing docs, and optionally code comments/refactoring.