Docs: Config generator: Open edit modal directly when adding new metadata

[JonasKellerer]

Currently, users have to click add, to add a new metadata field. Then they can click edit, to modify the data.

Change this, so when clicking the add button, a modal opens, which lets the user edit the metadata directly. If the user clicks in this modal 'OK' then the metadata should be added to the config. Otherwise the data will be discarded.