Add workflow to generate doxygen on tag push.

Took 17 minutes

.github/workflows/docs.yml

@@ -0,0 +1,31 @@
+name: Generate and Deploy Doxygen Docs
+
+on:
+  push:
+    tags:
+      - '*' # Only re-generate docs when a new version is pushed
+
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+        with:
+          submodules: true
+
+      - name: Install Doxygen and Graphviz
+        run: sudo apt-get install -y doxygen graphviz
+
+      - name: Generate Documentation
+        run: doxygen Doxyfile
+
+      - name: Deploy to cockatrice.github.io
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          deploy_key: ${{ secrets.DOCS_DEPLOY_KEY }}
+          external_repository: Cockatrice/cockatrice.github.io
+          publish_branch: main
+          publish_dir: ./html
+          destination_dir: docs   # Docs will live under https://cockatrice.github.io/docs/