Runner on k8s docs should point to the official Helm chart

The info in https://docs.gitlab.com/runner/install/kubernetes.html is deprecated and should be populated with what we have in https://docs.gitlab.com/ee/install/kubernetes/gitlab_runner_chart.html.

We can then redirect https://docs.gitlab.com/ee/install/kubernetes/gitlab_runner_chart.html to https://docs.gitlab.com/runner/install/kubernetes.html.

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Old description

Based on these two comments [1] and [2], I think we need to clarify the docs for installing a Runner on Kubernetes.