update(docs jest installation guide): add an example of loading jest.config.js

Author: ArturT

docusaurus/docs/jest/guide/index.mdx

@@ -808,6 +808,8 @@ If you are using Create React App, make sure to:
 
 <ShowIfSearchParam searchParam="ci">
 
+You need to explicitly tell Knapsack Pro to [load the `jest.config.js` file](../cookbook.md#use-a-jest-config-file) if needed.
+
 ### Verify that everything works
 
 Push a new commit to your repository and visit your [dashboard](https://knapsackpro.com/dashboard) to make sure all your CI nodes were recorded successfully in _Show build metrics > Show (build)_.