Remove Kubernetes overview from the Environment index page

What does this MR do and why?

This MR is part of a larger effort to move the cluster UI under the environment details page.

Previous MRs of this series:

• Add Deployments history tab on environments det... (!143865 - merged)
• Add Kubernetes overview tab on environments det... (!144370 - merged)

The current MR removes the Kubernetes overview from the Environment index page. It updates the related docs.

Note: a lot of files were moved and their imports were updated. I divided the work into 2 main commits:

• 6b881788 removes the unnecessary code;
• d17aae5b moves the files and updates their imports.

MR acceptance checklist

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Before	After

How to set up and validate locally

Prerequisites:

• NGINX, loopback interface, and HTTPS set up on your GDK
• Docker instance
• KAS enabled on GDK (steps 1-2 from the guide).

1. Visit the Project -> Operate -> Environments

2. Create/Edit an environment using the UI

3. Visit the Project -> Infrastructure -> Kubernetes clusters page and create an agent following the instruction from the modal.

   • Select the "Connect a cluster" button
   • The modal should pop up
   • In the modal select "Select an agent or enter a name to create new"
   • You probably won't have any configured agents to show up in the list, create a new one by typing the name of your choice
   • The button should appear at the bottom of the list saying "Create agent: <your-agent-name>"
   • Select the button and click "Register" in the next view.
   • Save the token to use it in the next point.

4. Add the following configuration inside your project in .gitlab/agents/<your-agent-name>/config.yaml for the user_access agent:

   user_access:
     access_as: 
       agent: {}
     projects:
     - id: <your-group>/<your-project-to-share-agent-with>

5. Note that the shared agents should be connected to the cluster in order to appear in the list. Please follow points 3-8 from the guide and then the Deploy the GitLab Agent (agentk) with k3d section to create a local cluster and connect your agent with the cluster.

6. Select an agent from the dropdown in the Environments settings page and save the change.

7. Visit the Environments page and expand the environment that has an associated agent.

8. Verify there's no Kubernetes overview for this environment.

Related to #431746 (closed)