Initial draft of feature_matureness.md

[jkwak-work]

Closes shader-slang/slang#4840

The content is written by @csyonghe in a shared document.

[csyonghe]

Let's move this file to shader-slang.github.io/docs for now, so it gets consistent formatting without website pages.

Ah, this is already in shader-slang.github.io, all good then.

[csyonghe]

Let's add a link to this doc from docs/index.md, probably at the bottom of the page.

[jkwak-work]

I made some changes based on the feedbacks.
It is ready for another review.