Amplify JS "Use Existing Resources" instructions confusing - Make parity with Swift and Android docs

**Describe the content issue**:
The instructions on how to use existing resources using the JavaScript library are sparse, and seem to discourage use without the CLI. The current docs for this method are https://docs.amplify.aws/lib/client-configuration/configuring-amplify-categories/q/platform/js/. This name does not indicate it is meant for existing resources, and it is hidden way down the page in a category of one item. The grand summary of the page is "If you are not using the Amplify CLI or need to override these settings, this documentation shows the available configuration properties for each category." The rest of the page is just snippets of configuration without any additional context.

This can and HAS been done better:
https://docs.amplify.aws/lib/project-setup/use-existing-resources/q/platform/ios/
https://docs.amplify.aws/lib/project-setup/use-existing-resources/q/platform/android/

Both of these pages explain clearly that the CLI is not required in the Project Startup category. They walk through creating the file. They then link to the relevant pages to set up the category configuration.

**URL page where content issue is**: https://docs.amplify.aws/lib/client-configuration/configuring-amplify-categories/q/platform/js/

Start with the JavaScript configuring-amplify-categories page linked in the issue and compare it with the iOS and Android use-existing-resources pages. Done means the JavaScript documentation clearly explains that the CLI is not required, walks through creating the configuration file, and links to the relevant category setup pages.

Written by the indexing model from the issue text.