added workflow to create pdf docs (experimental)

.github/workflows/pdf.yml

@@ -0,0 +1,19 @@
+name: Makefile CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - uses: xu-cheng/texlive-action/full@v1
+      
+    - name: Make manual
+      run: make pdf