chore: Add AI catalog page behind FF

What does this MR do and why?

Introduce feature flag :global_ai_catalog.

To get started on the duo platform page and duo catalog, we need a basic page setup so that we can iterate from there. This adds the basic functionalities expected:

1. Rails route
2. Rails controller
3. Single entry point
4. Vue application
5. Vue Router for SPA
6. VueApollo to enable fetching graphql data

References

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://127.0.0.1:3000/explore/
2. Verify that the menu does display AI Catalog item
3. In rails console enable the feature flag

   Feature.enable(:global_ai_catalog)

4. Visit http://127.0.0.1:3000/explore/ and veirfy that AI Catalog item is now displaying
5. Click on it to visit the new page

MR acceptance checklist

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