readme: switch to simpler github badges (no more shields)

Author: cagix

readme.md

@@ -21,9 +21,9 @@ has_license: true
 
 **Read Online**:
 
-[![GFM: Workflow Status](https://img.shields.io/github/actions/workflow/status/Programmiermethoden-CampusMinden/Prog2-Lecture/gfm.yaml?branch=master&style=flat&label=GitHub%20Preview)](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/blob/_gfm/readme.md)
-[![Docsify: Workflow Status](https://img.shields.io/github/actions/workflow/status/Programmiermethoden-CampusMinden/Prog2-Lecture/docsify.yaml?branch=master&style=flat&label=Docsify%20Preview)](https://docsify-this.net/?basePath=https://raw.githubusercontent.com/Programmiermethoden-CampusMinden/Prog2-Lecture/refs/heads/_docsify&homepage=readme.md&sidebar=true&browser-tab-title=PR2&font-size=14px&searchbox=true&externalLinkTarget=_blank&mergeNavbar=true&zoom-images=true&dark-mode=auto#/)
-[![PDF: Workflow Status](https://img.shields.io/github/actions/workflow/status/Programmiermethoden-CampusMinden/Prog2-Lecture/pdf.yaml?branch=master&style=flat&label=PDF-Version)](https://raw.githubusercontent.com/Programmiermethoden-CampusMinden/Prog2-Lecture/_pdf/readme.pdf)
+[![GitHub Preview](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/actions/workflows/gfm.yaml/badge.svg)](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/blob/_gfm/readme.md)
+[![Docsify Preview](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/actions/workflows/docsify.yaml/badge.svg)](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/blob/_docsify/readme.md)
+[![PDF-Version](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/actions/workflows/pdf.yaml/badge.svg)](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/blob/_pdf/readme.pdf)
 
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