docs: Add changelog entry for reportFullyDisplayed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonis

CHANGELOG.md

@@ -11,6 +11,7 @@
 ### Features
 
 - Add experimental `extendAppStart`/`finishExtendedAppStart`/`getExtendedAppStartSpan` to extend the standalone app start window and instrument post-init work ([#6392](https://github.com/getsentry/sentry-react-native/pull/6392))
+- Add `Sentry.reportFullyDisplayed()` imperative API for signaling Time to Full Display ([#6419](https://github.com/getsentry/sentry-react-native/pull/6419))
 
 ### Changes