Remove stable docs badge (no stable release yet)

Author: Sixzero

README.md

@@ -1,6 +1,5 @@
 # OpenRouterCLIProxyAPI.jl
 
-[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://sixzero.github.io/OpenRouterCLIProxyAPI.jl/stable/)
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://sixzero.github.io/OpenRouterCLIProxyAPI.jl/dev/)
 [![Build Status](https://github.com/sixzero/OpenRouterCLIProxyAPI.jl/actions/workflows/CI.yml/badge.svg?branch=master)](https://github.com/sixzero/OpenRouterCLIProxyAPI.jl/actions/workflows/CI.yml?query=branch%3Amaster)