docs: update data retention info

[marcel-rbro]

Resolves #1720

---

Note

Revamps storage usage docs to clarify plan-based data retention and adds step-by-step guidance to name storages for indefinite retention.

• Docs:
  • Data retention: Update policy to plan-based retention; free plan keeps last 10 runs up to 4 months; named storages retained indefinitely; unnamed storages deleted per plan.
  • Preserving storages: Replace paragraph with concise step-by-step UI flow (find ID, open details, Actions → Rename) and clearer API instructions; reposition supporting image.
  • Minor wording tweaks for clarity in sources/platform/storage/usage.md.

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[apify-service-account]

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