Use Sphinx for docs and improve Doxygen comments

This change makes the API documentation look much nicer.

Usage instructions were moved from README to docs.

Author: martinjaeger

.github/workflows/docs.yml

@@ -0,0 +1,34 @@
+name: Documentation
+
+on:
+  push:
+    branches: [ main ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0      # necessary to get tags
+
+      - name: Install dependencies
+        run: |
+          sudo apt install -y git make python3 python3-pip doxygen graphviz
+          pip3 install -r docs/requirements.txt
+
+      - name: Build documentation
+        run: |
+          cd docs
+          make html
+
+      - name: Deploy to gh-pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: docs/build/html/
+          enable_jekyll: false
+          allow_empty_commit: false
+          force_orphan: true
+          publish_branch: gh-pages

.github/workflows/doxygen.yml

@@ -36,4 +36,7 @@
 .vscode
 html
 
+# Zephyr unit-test build files
+build
+
 .thingset-shell-history.txt