Update demo workflow to trigger on dev-actions branch and add Python environment setup

.forgejo/workflows/demo.yaml

@@ -1,6 +1,11 @@
-on: [push]
+on:
+  push:
+    branches:
+      - "dev-actions"
 jobs:
   test:
     runs-on: self-hosted
     steps:
-      - run: echo All Good
+      - run: /usr/bin/python -m venv .env
+      - run: ./.env/bin/pip install myst
+      - run: ./.env/bin/python -m myst build --html