DOC add note about how to skip tests during CI
Tomorrow there will be a PyDataGlobal sprint, which'll probably involve many, many documentation-related pull requests, so this might help with not using up azure pipelines resources unnecessarily

EDIT
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doesn't seem to be working :flushed:
:thinking: would've expected this to work, from https://docs.microsoft.com/en-us/azure/devops/pipelines/repos/azure-repos-git?view=azure-devops&tabs=yaml#skipping-ci-for-individual-commits