Update monetization guide with modern purchase flows

[shai-almog]

Summary

• remove references to deprecated PurchaseCallback hooks and steer readers toward receipt-based entitlement checks
• document the promotional-offer overloads on purchase/subscribe along with the ApplePromotionalOffer helper
• describe the restore and subscription management entry points so developers can surface the native store UIs

Testing

• not run

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https://chatgpt.com/codex/tasks/task_e_68fb148285c08331b4bca86c8db824db

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF
• AsciiDoc linter report
• Vale report
• Unused image report

Developer Guide quality checks:

• AsciiDoc linter: No issues found (report)
• Vale: 18556 alert(s) (2727 errors, 6286 warnings, 9543 suggestions) (exit code 1) (report)
• Image references: No unused images detected (report)