Update workflow documentation for the single codebase

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In a variety of places we have documentation describing how to deal with the split between CE and EE, such as when to set up EE specific merge requests. We should update this documentation for the new single codebase setup, if not done so already. Unfortunately, apart from PROCESS.md I am not sure which files exactly we would have to change, and how. We also need to update any community contribution related documentation.