docs: example of age print column

[caarlos0]

Added an "Age" print column as an example as well.

Its a pretty common use case, and it was not obvious to me how to make it work. In my head, I would have to do something like now - .metadata.creationTimestamp somehow.... found that I just need to set the type to date by digging the prometheus-operator codebase.

Hope this makes it easier for the next people :)

[k8s-ci-robot]

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[caarlos0]

/assign @droot

[camilamacedo86]

/ok-to-test

[camilamacedo86]

It shows ok for me so
/approved

Let's waiting for a second reviewer ok.

[mengqiy]

/lgtm
/approve

[k8s-ci-robot]

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This pull-request has been approved by: caarlos0, mengqiy

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