DOC: update documentation build status badge (#239)

README.md

@@ -3,7 +3,7 @@
 [![10.5281/zenodo.6908149](https://zenodo.org/badge/DOI/10.5281/zenodo.6908149.svg)](https://doi.org/10.5281/zenodo.6908149)
 [![GPLv3+ license](https://img.shields.io/badge/License-GPLv3+-blue.svg)](https://www.gnu.org/licenses/gpl-3.0-standalone.html)
 
-[![Documentation build status](https://readthedocs.org/projects/pwa/badge/?version=latest)](https://pwa.readthedocs.io)
+[![CI](https://github.com/ComPWA/compwa.github.io/actions/workflows/ci.yml/badge.svg)](https://github.com/ComPWA/compwa.github.io/actions/workflows/ci.yml)
 [![Binder](https://static.mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/ComPWA/compwa.github.io/main)
 [![Google Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/ComPWA/compwa.github.io/blob/main)
 [![Open in Visual Studio Code](https://img.shields.io/badge/vscode-open-blue?logo=visualstudiocode)](https://open.vscode.dev/ComPWA/compwa.github.io)