docs: map plugin method return types to TypeScript

[riderx]

Summary

• Follow-up to Clarify return types for Capacitor iOS plugins #471. Return types belong on Method Types for all platforms, but that page used never and did not show what to declare in src/definitions.ts.
• Map each native returnType to the TypeScript signature (Promise<void>, Promise<T>, callback + Promise<CallbackID>), and note that never cannot be implemented in web.ts.
• Link the iOS and Android plugin guides to that page instead of duplicating the native return-type list.

Test plan

• Check Method Types shows the TypeScript ↔ Android ↔ iOS table and per-section definitions.ts snippets
• Confirm void methods are documented as Promise<void> (not never)
• Confirm iOS and Android creating-plugin guides link to Method Types

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[vercel]

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