Before forbidding auto-created fleets in #3059, the docs and examples should be updated to teach current and new users the fleet-first UX. This includes:

• Always create fleets for multi-node tasks.
• Include fleet configurations directly in examples – currently examples are preceded with "For more detials on how to use clusters with dstack, check the Clusters guide." notice but the Cluster guide itself has no fleet configuration examples.
• Update quickstart to create fleets before applying run configurations.
• Drop all mentions of autocreated fleets (see Creation policy here and here)