update docs - include deprection workflow

docs/schulcloud-server/Coding-Guidelines/deprection-workflow.md

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+# Deprecation Workflow
+
+## When to use 2 step migration
+
+When we are replacing code which is used in other modules we should use a 2 step migration.
+This is meant to prevent merge conflicts and to make it easier to review the changes.
+Also it makes it easier to find the code which needs to be changed in the other modules.
+Please note that this is not always possible, but should be used when possible.
+
+Step 1: Add new alternative code  
+Step 2: Mark the old code with "@deprecated" add a hint in the comments to use the new code.  
+Step 3: Inform team  
+Step 4: Remove deprecated code (when is hard to say, once all dependencies are removed)