docs: provide a simple and clear description how to use styles with DB UX

[annsch]

core-web/README.md

Line 39 in c7afcb6

Afterwards you need to add the specific CSS or SCSS files to provide the styles of DB UX Design System (Version 3), as described in the "Styling Dependencies" of each packages `README`.

e.g. link to each packages readme and/or add README for https://github.com/db-ux-design-system/examples (this is the linked file)