Document rehype-typst

[Loosetooth]

I discovered the existence of the rehype-typst package by accident because of #505

I tried it out using Next.js + MDX + rehype-typst in my example project here and the basic functions seem to work fine!

Perhaps it would be a good idea to add information about rehype-typst to the README, so that others can find it more easily.
Right now it's really hidden.

Let me know if you need any help.