docs: Enhance documents for Materials

[munechika-koyo]

Hi,

I would like to suggest enhancing the documents for 8.Materials page, not only by attaching images but also by including content about material properties (e.g., refractive index and extinction coefficient), Fresnel term, and so on.

If you're in favor, I will suggest some drafts of the page that can be created using raysect functionalities.