Convert documentation into sphinx-immaterial theme framework

[deepakmaroo]

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)