Add docs on user preferences

[cbrinicombe13]

Note

Adds first-class user notification preferences to the API and docs.

• New GET /users/{id}/preferences and PATCH /users/{id}/preferences endpoints with examples and SDK snippets
• New schemas: NotificationChannel, NotificationType, NotificationPreferences, UserPreferencesResponse, UpdateUserPreferencesRequest
• Updates platform/users, platform/emails, and platform/push-notifications to link and explain preference center usage; adds snippets/*-user-notification-preferences-block.mdx
• Registers new endpoints in docs.json
• Minor formatting in api-reference/openapi.yml (quote normalization)

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