Kubernetes dashboard state with no agent ID provided

What does this MR do and why?

This MR adds an index page for the Kubernetes dashboard to show a state where no agent was provided.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the k8s_dashboard feature flag

      Feature.enable(:k8s_dashboard)

1. Visit https://gdk.test:3443/-/kubernetes/ and verify the page

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 #428310 (closed)