implement HTML API reference autogeneration

This adds a jinja template to generate markdown files
(with lots of HTML mixed in) from YAML using the existing
interface_generator.py script.

The docs website layout files are modified to incorporate the
new documentation.

While previously Github Pages was automatically running
Jekyll on the docs folder, this commit adds a custom Github
workflow to compile and deploy the website to facilitate
the custom markdown generation step before Jekyll runs.

.github/workflows/documentation.yml

@@ -0,0 +1,67 @@
+name: Build and publish HTML documentation website
+
+on:
+  push:
+    branches: [ feature/doc_autogen ]
+
+jobs:
+  jekyll:
+    runs-on: ubuntu-16.04
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.x'
+
+    # Use GitHub Actions' cache for ruby and python packages to shorten build times and decrease load on servers
+    - name: Cache gems
+      uses: actions/cache@v2
+      with:
+        path: docs/vendor/bundle
+        key: ${{ runner.os }}-gems-${{ hashFiles('docs/Gemfile.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-gems-
+    
+    - name: Cache pip
+      uses: actions/cache@v2
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-PyYAML-Jinja2-jsonschema
+        restore-keys: |
+          ${{ runner.os }}-pip-
+          ${{ runner.os }}-
+
+    - name: Install Python dependencies
+      run: pip install PyYAML Jinja2 jsonschema
+
+    # Autogenerate the API reference .md files in the python in the python/python3 container
+    - name: Autogenerate the API reference .md files in the python container
+      run: |
+        mkdir -p docs/_api docs/_includes
+        python Firmware/interface_generator_stub.py --definitions Firmware/odrive-interface.yaml --template docs/_layouts/api_documentation_template.j2 --outputs docs/_api/#.md
+        python Firmware/interface_generator_stub.py --definitions Firmware/odrive-interface.yaml --template docs/_layouts/api_index_template.j2 --output docs/_includes/apiindex.html
+    
+    - name: Build the site in the jekyll/builder container
+      run: |
+        docker run \
+        -v ${{ github.workspace }}:/srv/jekyll -e PAGES_REPO_NWO=${GITHUB_REPOSITORY} \
+        ruby:2.7-buster /bin/sh -c "
+          chmod 777 /srv/jekyll/docs && \
+          cd /srv/jekyll/docs && \
+          bundle config path vendor/bundle && \
+          bundle install && \
+          JEKYLL_ENV=production bundle exec jekyll build
+        "
+        touch .nojekyll
+
+    - name: Push to documentation branch
+      run: |
+        git config user.name "${GITHUB_ACTOR}"
+        git config user.email "${GITHUB_ACTOR}@users.noreply.github.com"
+        git add -f docs/_site
+        git commit -m "jekyll build from Action ${GITHUB_SHA}"
+        git push --force origin HEAD:${REMOTE_BRANCH}
+      env:
+        REMOTE_BRANCH: gh-pages