docs: add Windows quickstart and offline dry-run examples

[aydnOktay]

This PR improves the documentation around offline data generation and adds a dedicated Windows quickstart section.

What’s included

• Offline Data Generation Quick Start
  • Clarifies the recommended call order for starting run-api, an environment, and the offline generators.
  • Adds an optional --dry-run step to atropos-sft-gen so users can validate their configuration before collecting SFT data:
    atropos-sft-gen path/to/output.jsonl
    --tokenizer Qwen/Qwen2.5-1.5B-Instruct
    --dry-run