Add API key GIF to JustCall provider guide

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: ejoo

src/images/provider-guides/justcall-api-key.gif

@@ -46,4 +46,6 @@ To integrate with JustCall:
 2. Navigate to **Settings** > **API Keys** (or go directly to [API Settings](https://app.justcall.io/app/settings/api)).
 3. Generate a new API key and secret pair.
 
+![JustCall API Key](/images/provider-guides/justcall-api-key.gif)
+
 For more details, see the [JustCall Authentication documentation](https://developer.justcall.io/reference/authentication).