docs: Update Android WebView guidance for connected traces #6161

Outcome

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

Acceptance criteria

• Documents SentryWebViewTracing.addTracePropagation(webView, allowedHosts...). It writes the native trace into the page as sentry-trace and baggage meta tags at document start, using androidx.webkit's WebViewCompat.addDocumentStartJavaScript.
• Documents the prerequisites:
  • the app adds the androidx.webkit:webkit dependency;
  • the System WebView supports document-start scripts (minimum version TBD);
  • the call is made on the main thread, before loadUrl;
  • otherwise the call logs a warning and returns false.
• Documents allowedHosts as a trust boundary, separate from tracePropagationTargets:
  • matching covers the host and its subdomains, over https;
  • a non-default port in an entry is kept.
• Documents that the page in the WebView must run @sentry/browser with browserTracingIntegration.
• Documents the limitations:
  • the trace is a snapshot taken at call time, so call it again to update it for a later page load;
  • only the main frame receives it, and only the entry pageload continues the trace;
  • sentry-transaction is removed from baggage;
  • native and web sampling can differ when no span is active;
  • Custom Tabs and loadData/file:// content aren't supported (use WebViewAssetLoader for bundled pages).
• Wording coordinated with the Apple and React Native WebView doc updates for cross-platform consistency.

References

• Implementation: feat(android): Connect traces with web content in a WebView #6395. The sample screen is sentry-samples-android's WebViewActivity.
• Meta-tag continuation contract the page should point to: https://docs.sentry.io/platforms/javascript/tracing/distributed-tracing/
• Current Android WebView page to replace: https://docs.sentry.io/platforms/android/configuration/webview/
• Cross-platform parity: RN docs issue (docs: Update React Native WebView guidance for connected traces sentry-react-native#6782), Apple docs issue