Add missing xsltproc

Author: marc1706

.github/workflows/tests.yml

@@ -56,6 +56,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - name: Install xsltproc
+        run: |
+          sudo apt-get install -y xsltproc
       - name: Build documentation for website
         working-directory: ${{ github.workspace }}/documentation
         run: |