Added github workflow to build C++ documentation

Author: andrewherren

.github/workflows/docs.yml

@@ -0,0 +1,66 @@
+# Combination of https://github.com/RobinMagnet/pyFM/blob/master/.github/workflows/documentation.yml
+# https://github.com/r-lib/pkgdown/blob/main/.github/workflows/pkgdown.yaml
+# and https://github.com/DenverCoder1/doxygen-github-pages-action
+name: docs
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  release:
+    types: [published]
+  workflow_dispatch:
+
+jobs:
+  build_documentation:
+    name: Build Documentation
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: write
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+      with:
+        repository: 'StochasticTree/stochtree'
+        ref: 'cpp_docs'
+        submodules: 'recursive'
+
+    - name: Setup Python 3.10
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3.10"
+        cache: "pip"
+
+    - name: Install Package with Relevant Dependencies
+      run: |
+        pip install --upgrade pip
+        pip install -r cpp_docs/requirements.txt
+        pip install .
+    
+    - name: Install doxygen and graphviz
+      run: |
+        sudo apt-get install doxygen graphviz -y
+    
+    - name: Build doxygen XML files
+      run: |
+        doxygen cpp_docs/Doxyfile
+
+    - name: Build HTML
+      run: |
+        sphinx-build -M html cpp_docs/ cpp_docs/_build/
+
+    - name: Upload Artifact
+      uses: actions/upload-artifact@v4
+      with:
+        path:
+          cpp_docs/_build/html/
+    
+    - name: Deploy to GitHub Pages
+      uses: peaceiris/actions-gh-pages@v4
+      if: github.ref == 'refs/heads/main'
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: cpp_docs/_build/html