Quick heads-up: stable vs. unstable docs

[hishamhm]

Teal documentation now points to the latest stable release: https://teal-language.org/book/latest/index.html

The latest docs generated from main are now live at: https://teal-language.org/book/unstable/index.html

This is to avoid confusion from people installing tl and then seeing features in the docs that are not available in the stable compiler.