Replace demo workflow with serve workflow, adding dependencies and enhancing file copy process

.forgejo/workflows/serve.yaml

@@ -0,0 +1,20 @@
+on:
+  push:
+    branches:
+      - "main"
+jobs:
+  test:
+    runs-on: self-hosted
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Initialize virtual environment
+        run: /usr/bin/python -m venv .env
+      - name: Install dependencies
+        run: ./.env/bin/pip install mystmd jupyterlab_myst ipykernel altair pandas
+      - name: Build static HTML
+        run: ./.env/bin/myst build --html
+      - name: Copy files
+        run: |
+          rm -rf /data/srv/forgejo-runner/cours4.0
+          cp -r _build/html /data/srv/forgejo-runner/cours4.0