adding custom pages to plasma-welcome

we could make a proper bazaar page with this 

POC: https://github.com/renner0e/aurora/tree/welcome

Docs:
https://invent.kde.org/plasma/plasma-welcome

<img width="1013" height="948" alt="Image" src="https://github.com/user-attachments/assets/c0a37242-b9fd-47a7-9df8-24110434aa42" />

https://youtu.be/_xht6TvyuGA?si=EvtgZBu6YnzVhwOH&t=158