Document lastEventId with User Feedback

[buenaflor]

Description

Part of an initiative to get this supported and documented by all SDKs in the same way, relevant for User Feedback: getsentry/sentry-docs#9232

Add/link to docs under https://docs.sentry.io/platforms/dart/user-feedback/ and https://docs.sentry.io/platforms/flutter/user-feedback/

Consider JS deprecating lastEventId and how to sync it with native