feat: Document the User Feedback API

src/docs/sdk/features.mdx

@@ -75,9 +75,36 @@ Ability to get the ID of the last event sent. Event IDs are useful for correlati
 
 ## User Feedback
 
-On user-facing platforms such as mobile or the browser this means first class support for requesting user feedback when an error occurs.
+For all SDKs, it is strongly recommended to send the `User Feedback` as an [envelope](/sdk/envelopes/#user-feedback). Alternatively, the SDKs can
+use the [User Feedback endpoint](https://docs.sentry.io/api/projects/submit-user-feedback/), which is not recommended.
 
-On backend platforms, SDKs should document how to use the last event ID to prompt the user for feedback themselves.
+### User Facing Platforms
+On user-facing platforms such as mobile, desktop, or browser this means first-class support for requesting User Feedback when an error or crash occurs.
+To see some examples of the API check out the user-facing docs for [Apple](https://docs.sentry.io/platforms/apple/enriching-events/user-feedback/) and
+[Java](https://docs.sentry.io/platforms/java/enriching-events/user-feedback/).
+
+On mobile and desktop, it is common to prompt the user for feedback after a crash happened on the previous run of the application. Therefore the SDKs should
+implement the `onCrashedLastRun` callback on the options. This callback gets called shortly after the initialization of the SDK when the last program execution
+terminated with a crash. The SDK should execute the callback only once during the entire run of the program to avoid multiple callbacks if there are multiple 
+crash events to send.
+
+### Backend Platforms
+
+On backend platforms, SDKs should document how to use the [last event ID](#retrieve-last-event-id) to prompt the user for feedback themselves. 
+
+### Example implementations
+
+User Feedback class:
+ * [Objective-C](https://github.com/getsentry/sentry-cocoa/blob/9eedc425727f0daccca1bd2be6021f6c3d9c654c/Sources/Sentry/SentryUserFeedback.m)
+ * [Java](https://github.com/getsentry/sentry-java/blob/671f9e0b8b709ef18e8b0788c31df44f99d09d35/sentry/src/main/java/io/sentry/UserFeedback.java#L7)
+ * [C#](https://github.com/getsentry/sentry-dotnet/blob/6ddf8363170bffee74a8c3cbf44335797d39ca9c/src/Sentry/Protocol/UserFeedback.cs#L10)
+
+Envelope item:
+
+ * [Objective-C](https://github.com/getsentry/sentry-cocoa/blob/fd3e46efe59324e894d5601a7b3028824a588dda/Sources/Sentry/SentryEnvelope.m#L145-L162)
+ * [Java](https://github.com/getsentry/sentry-java/blob/b2e8dd43ea2ef37201b215ec409655e49e3231c8/sentry/src/main/java/io/sentry/SentryEnvelopeItem.java#L131-L155)
+ * [C#](https://github.com/getsentry/sentry-dotnet/blob/3bc6c66a14e653982a6a25df033d68d8ad12c4af/src/Sentry/Protocol/Envelopes/EnvelopeItem.cs#L170-L178)
+
 
 ## Before-Send Hook