docs(starter): explain adding an API reference

docs/content/api-references.mdx

@@ -1,6 +1,16 @@
 # API References
 
-Scalar supports three ways to add OpenAPI documents to your documentation: using a local file, the [Scalar Registry](https://scalar.com/products/registry/getting-started) or a remote URL.
+Add an API reference from the Scalar dashboard, or configure a local file, a Scalar Registry document, or a remote URL in this repository.
+
+## Using the Scalar Dashboard
+
+1. Open your docs project in the Scalar dashboard.
+2. Click the **+** button next to the docs navigation heading and select **Add API Reference**.
+3. Select an existing API from your team and click **Add API**. To use a new document, select **Import a new API** and click **Continue**. If your team has no APIs yet, the import screen opens directly.
+4. When importing, upload or paste your OpenAPI or AsyncAPI document. Review its Registry path and choose **Public** or **Private** access, then click **Add API**.
+5. Open the new API reference in the docs sidebar to check its content, then save your changes.
+
+Importing stores the document in the Scalar Registry and adds a reference to your docs navigation. Importing an API into the Registry on its own does not add it to a docs project. Use **Add API Reference** to attach it.
 
 ## Using a Local File