Add execution step to Forgejo workflow for MyST build

.forgejo/workflows/serve.yaml

@@ -14,6 +14,10 @@ jobs:
         run: /usr/bin/python -m venv .venv
       - name: Install dependencies
         run: ./.venv/bin/pip install -r requirements.txt
+      - name: Execute code
+        run: |
+          . .venv/bin/activate
+          myst build --execute
       - name: Build PDF exports
         run: |
           . .venv/bin/activate