Custom icons in repeating groups

### Description

The setup doc for repeating groups describes how to customize button texts, e.g. for "add", "save", "edit" etc.

I have a use case where the user should be presented with some data, but is not allowed to edit. So I have hidden the "Add new" and "Delete" buttons, and changed the texts from "edit" and "save" to "show" and "close". But the icons for those two buttons is a pencil. Pencil works fine with "edit" and (perhaps to a lesser extent) "close", but it looks strange for "show" and "close".

Would be nice if the button icons were also customizable, like the texts.

https://docs.altinn.studio/app/development/ux/fields/grouping/setup/

### Additional Information

_No response_

Start with the repeating-groups setup documentation linked in the issue and trace the existing customization for button text in the frontend. Done means repeating-group button icons can be customized for actions such as show and close, with the available configuration documented.

Written by the indexing model from the issue text.