Add README badge linking to stable documentation (#339)

* Add docs badge to link to stable docs

* Add ruff badge

* Remove ruff

README.md

@@ -7,6 +7,7 @@
   [![Qiskit](https://img.shields.io/badge/Qiskit-%E2%89%A5%200.43.0-6133BD)](https://github.com/Qiskit/qiskit)
   [![Qiskit Nature](https://img.shields.io/badge/Qiskit%20Nature-%E2%89%A5%200.6.0-6133BD)](https://github.com/Qiskit/qiskit-nature)
 <br />
+  [![Docs (stable)](https://img.shields.io/badge/Docs-stable-blue.svg)](https://qiskit-extensions.github.io/circuit-knitting-toolbox/)
   [![DOI](https://zenodo.org/badge/543181258.svg)](https://zenodo.org/badge/latestdoi/543181258)
   [![License](https://img.shields.io/github/license/Qiskit-Extensions/circuit-knitting-toolbox?label=License)](LICENSE.txt)
   [![Code style: Black](https://img.shields.io/badge/Code%20style-Black-000.svg)](https://github.com/psf/black)