docs: added README for DB UI to DB UX migration

Added documentation for migrating from DB UI to DB UX Design System v3.

Author: mfranzke

docs/migration/db-ui/README.md

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+# Migration from DB UI to DB UX Design System v3
+
+This folder contains the documentation on how to migrate from DB UI to DB UX Design System v3.
+
+If you are even already on our current Design System generation, have a look at the [GitHub Releases page](https://github.com/db-ux-design-system/core-web/releases) for the `CHANGELOG.md` file in each Node package for detailed information on changes.