docs: Update React Native WebView guidance for connected traces

[antonis]

Outcome

Replace the current React Native WebView guidance — which states the SDK "doesn't interact with the mobile application's web views" — with setup for the new trace-connection integration.

Acceptance criteria

• Documents the integration, the allowedHosts option, the requirement that the in-WebView page run @sentry/browser, and the entry-pageload scope limitation.
• Wording coordinated with the Cocoa/Android WebView doc updates for cross-platform consistency.

References

• Existing meta-tag continuation contract the page should point to: https://docs.sentry.io/platforms/javascript/tracing/distributed-tracing/
• Current RN WebView page to replace: https://docs.sentry.io/platforms/react-native/

[linear-code]

RN-799