Document `lastEventId` with User Feedback

[bruno-garcia]

The User Feedback docs for SDKs (other than React Native and PHP) we have no reference of lastEventId.

This is useful when using the API in the SDK as well as the crash report modal, on server side rendered pages.

• Document lastEventId with User Feedback sentry-java#3236
• Python: https://docs.sentry.io/platforms/python/user-feedback/
• Unity: https://docs.sentry.io/platforms/unity/user-feedback/
• Unreal: https://docs.sentry.io/platforms/unreal/user-feedback/
• Document lastEventId with User Feedback sentry-dart#1902
• Implement & Document lastEventId with User Feedback sentry-kotlin-multiplatform#193
• Implement + document lastEventId sentry-cocoa#3691

Relates to:

• Add crash report API instructions for pure backend platforms on UF sidebar sentry#65728