docs: add data availability and gateway expectations guidance

[kempsterrrr]

Summary

Adds documentation for data availability and production gateway expectations, and updates navigation and cross-links for a clearer Learn to Build flow.

Changes

• Added content/learn/gateways/data-availability.mdx with mode-based guarantees and access strategy recommendations.
• Added content/build/access/gateway-expectations.mdx with production quick-start, gateway introspection guidance, and real-world turbo-gateway scenarios.
• Updated Learn gateway ordering so Data Availability is between Data Retrieval and Data Verification.
• Updated cross-link wording in Learn and Build access pages to emphasize production decision flow.
• Kept environment variable defaults aligned with upstream:
  • ON_DEMAND_RETRIEVAL_ORDER
  • BACKGROUND_RETRIEVAL_ORDER
  • X_402_USDC_NETWORK
  • X_402_USDC_FACILITATOR_URL

Validation

• Re-checked live endpoints:
  • https://turbo-gateway.com/ar-io/info
  • https://arweave.net/ar-io/info
• Ran yarn lint (passes with one pre-existing warning outside this change set).
• Ran yarn check-links (fails on pre-existing unrelated broken links).

Notes

Documentation-only PR. No runtime or API behavior changes.

[github-actions]

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Your preview is available at: https://docs-pr-107_docs.ar.io

Property	Value
Transaction ID	PCpdyS7bId0ZhTAulgp01ceK6CbwVKM6M7LGZhfGhwM
ArNS Name	docs
Undername	docs-pr-107

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