Add route and nav for Global CI/CD Catalog behind FF

What does this MR do and why?

This is introducing a FF global_ci_catalog which "moves" the CI/CD catalog from the namespace scope to become global. This means that we will have only one catalog that user can filter through instead of multiples based on the namespaces.

This MR:

• Adds the new route /base-url/explore/catalog/ and /base-url/explore/catalog/:id
• Adds a new FF
• Behind that flag, we add a new Nav item to the Explore menu
• Clicking on the link takes you to the empty page for now

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to the Explore menu
2. Notice the Menu item is not visible
3. Open your rails console
4. Run Feature.enable(:global_ci_catalog)
5. refresh the page on the explore menu
6. Notice the CI/CD Catalog menu item appears
7. Click on it
8. Notice you re taken to explore/catalog

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #427938 (closed)