Add C++ API documentation

- add GitHub Actions workflow for deploying pages
- add setup for sphinx + breath to format Doxygen generated documentation
- add pages for molecule, basis set, grid, runtime environment,
  load balancer, xc integrator and macros
- update header files with doxygen-style comments to display documentation

Author: awvwgk

.github/workflows/docs.yml

@@ -0,0 +1,64 @@
+name: Docs
+
+on:
+  push:
+    branches: [master, docs]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+          cache: 'pip'
+
+      - name: Install dependencies
+        run: pip install -r docs/requirements.txt
+
+      - name: Install Doxygen
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y doxygen
+
+      - name: Generate Doxygen
+        run: |
+          doxygen Doxyfile
+        working-directory: ./docs
+
+      - name: Build documentation
+        run: |
+          sphinx-build -b dirhtml docs docs/_build/html
+          touch docs/_build/html/.nojekyll
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs/_build/html
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deploy.outputs.page_url }}
+
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deploy
+        uses: actions/deploy-pages@v4

docs/.gitignore

@@ -0,0 +1 @@
+_doxygen/