Update docs badge

The docs badge links to the stable version of the documentation, not some in-development version. Thus, it makes sense to reflect this in the badge itself, I think.

README.md

@@ -1,6 +1,6 @@
 # HOHQMesh
 
-[![Docs-dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://trixi-framework.github.io/HOHQMesh)
+[![Docs-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://trixi-framework.github.io/HOHQMesh)
 [![Build Status](https://github.com/trixi-framework/HOHQMesh/workflows/CI/badge.svg)](https://github.com/trixi-framework/HOHQMesh/actions?query=workflow%3ACI)
 [![Coveralls](https://coveralls.io/repos/github/trixi-framework/HOHQMesh/badge.svg?branch=main)](https://coveralls.io/github/trixi-framework/HOHQMesh?branch=main)
 [![Codecov](https://codecov.io/gh/trixi-framework/HOHQMesh/branch/main/graph/badge.svg)](https://codecov.io/gh/trixi-framework/HOHQMesh)