Remove Flux landing page

Add navigation item to global navigation

Now that the content under flux.md has been incorporated into the main GitOps page and Flux tutorial, we can redirect the old Flux page and remove the nav item.

• Link to the merge request that removed the new page: gitlab!125743 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the updated nav: https://pw-remove-flux-nav-entry.docs.gitlab-review.app/ee/user/clusters/agent/gitops.html