Add proper docs for Advanced Endpoints > Object Storage (#241)

This is basically a rephrase of what we have in
https://docs.svix.com/stream/sinks/s3, but using webhooks vocabulary and
generalizing over all object storage sinks.

Preview at
https://svix-docs-git-lucho-os-docs-advendp-svix.vercel.app/advanced-endpoints/object-storage

Closes svix/monorepo-private#11402

Author: svix-lucho

docs/advanced-endpoints/object-storage.mdx

@@ -14,4 +14,67 @@ When **Advanced Endpoint Types** is [enabled](/advanced-endpoints/intro#enabling
 
 They will be able to configure the connection right in the App Portal.
 
-![Object Storage Endpoint Create](../img/advanced-endpoints/s3-configuration.png)
+![Object Storage Endpoint Create](../img/advanced-endpoints/s3-configuration.png)
+
+## Usage
+
+By default, all Object Storage Endpoints come bundled with the following transformation code.
+
+```JavaScript
+/**
+ * @param input - The input object
+ * @param input.events - The array of events in the batch. The number of events in the batch is capped by the Object Storage Endpoint's batch size.
+ * @param input.events[].payload - The message payload (string or JSON)
+ * @param input.events[].eventType - The message event type (string)
+ *
+ * @returns Object describing what will be put to the bucket.
+ * @returns returns.config
+ * @returns returns.config.format - The format of the request object put to the bucket. Valid values are "jsonl", "json", or "raw" (Defaults to jsonl).
+ * @returns returns.config.key - The name of the object that will be put to the bucket. This will be suffixed with a timestamp to avoid duplicate object names.
+ * @returns returns.data - The array of events to send to the bucket. This will be formatted according to the format.
+ */
+function handler(input) {
+    return {
+        config: {
+            format: "jsonl",
+            key: "object-generated-by-svix"
+        },
+        data: input.events
+    }
+}
+```
+
+`input.events` is the list of webhooks received by the endpoint, processed in batches. 
+
+`config` describes the object put in the destination - the key name of the object, and the format of the object saved to the bucket.
+
+By default, the actual object key is always suffixed with a timestamp after the transformations are run. This ensures each event batch is saved as a unique object in the bucket.
+
+For example, if the endpoint receives the following events:
+
+```json
+{
+  "eventType": "user.created",
+  "payload": "{\"email\": \"joe@enterprise.io\"}"
+}
+```
+
+```json
+{
+  "eventType": "user.login",
+  "payload": "{\"id\": 12, \"timestamp\": \"2025-07-21T14:23:17.861Z\"}"
+}
+```
+
+The default transformation code would result in the following object being uploaded to the bucket.
+
+![s3-recent-events](../img/stream/s3-example.png)
+
+And the files contents would match the jsonl format.
+
+```jsonl
+{"payload":{"email":"joe@enterprise.io"},"eventType":"user.created"}
+{"payload":{"id":12,"timestamp":"2025-07-21T14:23:17.861Z"},"eventType":"user.login"}
+```
+
+If you want to control the format of the object more precisely, you can use `config.format = "raw"`, and set `data` to a string of the exact file contents you want.