Improve docs structure (#214)

Co-authored-by: Jerry Ling <proton@jling.dev>

Author: tamasgal

README.md

@@ -7,6 +7,7 @@
 [![All Contributors](https://img.shields.io/badge/all_contributors-8-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 [![JOSS](https://joss.theoj.org/papers/bab42b0c60f9dc7ef3b8d6460bc7229c/status.svg)](https://joss.theoj.org/papers/bab42b0c60f9dc7ef3b8d6460bc7229c)
+[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliahep.github.io/UnROOT.jl/stable)
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliahep.github.io/UnROOT.jl/dev)
 [![CI](https://github.com/JuliaHEP/UnROOT.jl/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/JuliaHEP/UnROOT.jl/actions/workflows/ci.yml)
 [![pkgeval](https://juliahub.com/docs/General/UnROOT/stable/pkgeval.svg)](https://juliahub.com/ui/Packages/General/UnROOT)