Update documentation in rancher/turtles-docs to reflect Fleet usage

[mjura]

Update documentation in rancher/turtles-docs to reflect Fleet usage

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[furkatgofurov7]

@mjura shall we transfer this issue to turtles-docs repo?

[furkatgofurov7]

Unassigning for now since this needs to be documented after we replace it everywhere in the codebase first.