docs: refresh IPFS onboarding for new readers

[ootakazuhiko]

Summary

• replace the unavailable new-reader Infura IPFS prerequisite with a current Public IPFS onboarding path
• separate provider-neutral CID, pinning, ipfs://, and gateway responsibilities from the Pinata Web App example
• use a non-self-referential two-step image-folder / metadata-folder CID flow
• document server-side credential boundaries, scoped keys, signed upload URLs, and public-data handling
• add dated primary-source notes and static regression checks

Closes #132

Verification

• actionlint
• npm run check:all
• npm run test:foundry
  • positive invariant: 128 runs / 4096 calls / 0 reverts
  • intentional 1 wei mutation: detected
• book-formatter at audited SHA 69eb5c12f5a750b65614bc9bbbc3d7abd5aa6f6c
  • Unicode: 0
  • textlint: 0
  • links: 0
  • layout risk: 0
  • Markdown structure: 0
• Jekyll 4.4.1 build and rendered Day11 marker smoke check
• all seven primary-source URLs: HTTP 200
• git diff --check

Security and external-service boundary

No provider account, API key, JWT, private material, or credentialed upload was used. The onboarding route was validated against official IPFS, Pinata, and Infura documentation current on 2026-07-23 Asia/Tokyo. Service UI, plans, rate limits, gateway hosts, and access restrictions remain freshness-dependent and have explicit recheck conditions in the chapter.

[ootakazuhiko]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: 0017877d49

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