Convert documentation into sphinx-immaterial theme framework #55
Description
Key theme features:
- Navigation: sections, instant loading, scroll-to-top
- TOC: sticky + follow-scroll behaviour
- Colour palette: supports light (indigo/green) and dark (light-blue/lime) mode toggle
- GitHub link: repo icon linking back to the IMAS-Core GitHub repo
- Custom admonitions:
- output (purple, terminal icon)
- installation (blue, download icon)