Add documentation for kluctl-controller metrics

[aljoshare]

We need to add documentation for the new kluctl-controller metrics. This is a follow-up task for kluctl/flux-kluctl-controller#90.

[codablock]

Best is to somehow integrate the documentation into https://github.com/kluctl/flux-kluctl-controller/tree/main/docs and then let the www-kluctl.io sync (here) handle it.