add custom authentication details to generic http endpoint doc (#375)

Author: crn4

src/pages/how-to/stream-activity-to-generic-http.mdx

@@ -31,6 +31,11 @@ This tab is for the essential endpoint details.
     -   **No Authentication**: Sends requests without an `Authorization` header.
     -   **Bearer Token**: Adds an `Authorization: Bearer <your-token>` header to each request. You will need to provide your API key or token.
     -   **Basic Auth**: Adds an `Authorization: Basic <credentials>` header. You must provide the credentials in the format `username:password`.
+    -   **Custom Authentication**: Allows you to specify a custom HTTP header for authentication. You will need to provide both the header name (e.g., `X-API-Key`) and its value.
+
+<Note>
+After saving the integration, the configured authentication details will be converted into an HTTP header and will appear in the **Headers** tab.
+</Note>
 
 ### HTTP Headers (Optional)