Document CI/CD kubectl usage when self-signed KAS

Timo Furrerrequested to merge
master-patch-fcb7 into master

What does this MR do and why?

This change documents how to use kubectl in a CI job with the injected KUBECONFIG.

This is a follow up of Improve communication regarding how to install ... (gitlab-org/charts/gitlab#3839 - closed)

and basically documents what's described in Support injecting the certificate for self sign... (#366955)

Closes Improve communication regarding how to install ... (gitlab-org/charts/gitlab#3839 - closed)

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