docs(stripe): sync Initialize guide with upstream iOS return URL flow

[rdlabo]

Summary

• Sync Stripe initialize.md (EN/JA) with the latest upstream documentation from capacitor-community/stripe main.
• Add iOS redirect-based payment setup: custom URL scheme in Info.plist, appUrlOpen listener forwarding, and updated handleURLCallback guidance.
• Regenerate the Japanese Stripe generated page.

Test plan

• npm run fmt:check
• npm run lint
• npm test
• Verified initialize.md fenced code blocks match byte-for-byte between EN, JA, and upstream EN

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