Trial Github Action to test PDF compilation

Part of #345 - just as a test alongside Circle CI (which currently fails
due to missing SSH key) and without moving the example PDFs to releases.

.github/workflows/main.yml

@@ -0,0 +1,16 @@
+name: Compile PDFs
+
+on:
+  push: {}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container: thomasweise/texlive
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+
+      - name: Compile
+        run: make