Add docs for afterCreate backend hook

[jpunke]

I was working on a development setup that syncs workflows from n8n into my repo. I was using the workflow.afterUpdate hook for that. This one only triggers when an existing workflow is saved, though. I tried using the documented workflow.create hook, but of course, the workflow hasn't been fully created yet; thus, no workflow ID to match on later exists.

Using the undocumented workflow.afterCreate hook solved this nicely for me.

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