Update DORA doc with info about deployment tier via the deployment API

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Related to this email:

Hi Haim and Ariel!

The tier variable is specified for the given environment and not for the deployments. If you create a deployment via the API, make sure you set the environment name where you have the deployment tier configured. https://docs.gitlab.com/ee/api/deployments.html#create-a-deployment


Regards,
Adam Hegyi

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