UI for additional URLs

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Currently I either have to use a XC to create additional URLs or create custom fields `KP2A_URL`, `KP2A_URL_1` etc manually.

### Describe the solution you'd like:

It would be nice if we can do that easily from the app. Thanks

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

No files or tests are named. Start by reviewing the app's entry-editing UI and the existing handling of XC-created URLs and KP2A_URL custom fields; define how users should add and manage multiple additional URLs. Done means additional URLs can be created easily from the app without manually adding custom fields.

Written by the indexing model from the issue text.