Document how to use the OpenShift certificate manager

[cescoffier]

OpenShift comes with its own version of the cert manager, based on annotations. See https://docs.openshift.com/container-platform/4.16/security/certificates/service-serving-certificate.html.

This PR extends the documentation to cover this OpenShift feature.

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[gsmet]

Nice content!

Not sure why you dropped this space but it needs to be added back.

For the titles, probably something to do in another PR someday, I just wanted to draw your attention on it for the future.

Please dismiss when space if fixed!

[quarkus-bot]

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It should be safe to merge provided you have a look at the other checks in the summary.

[github-actions]

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