Add vnu-jar for HTML validation.

Also, add a GitHub Action which builds the docs and run the validator script.

.github/workflows/docs.yml

@@ -0,0 +1,73 @@
+name: Docs
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - "**"
+
+env:
+  CI: true
+  NODE: 12.x
+  RUBY: 2.6.x
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Clone repository
+        uses: actions/checkout@v2
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: "${{ env.NODE }}"
+
+      - name: Set up Ruby
+        uses: actions/setup-ruby@v1
+        with:
+          ruby-version: ${{ env.RUBY }}
+
+      - name: Disable gem docs
+        run: 'echo "gem: --no-document" > ~/.gemrc'
+
+      - name: Set up npm cache
+        uses: actions/cache@v2
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-v${{ env.node }}-${{ hashFiles('package.json') }}-${{ hashFiles('package-lock.json') }}}
+          restore-keys: |
+            ${{ runner.OS }}-node-v${{ env.node }}-${{ hashFiles('package.json') }}-${{ hashFiles('package-lock.json') }}
+            ${{ runner.OS }}-node-v${{ env.node }}-
+
+      - name: Set up Ruby cache
+        uses: actions/cache@v2
+        with:
+          path: vendor/bundle
+          key: ${{ runner.os }}-ruby-v${{ env.RUBY }}-${{ hashFiles('Gemfile') }}-${{ hashFiles('Gemfile.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-ruby-v${{ env.RUBY }}-${{ hashFiles('Gemfile') }}-${{ hashFiles('Gemfile.lock') }}
+            ${{ runner.os }}-ruby-v${{ env.RUBY }}-
+
+      - name: Set up Bundler
+        run: gem install bundler -v "~> 1.17"
+
+      - run: ruby --version
+      - run: gem --version
+      - run: bundle --version
+      - run: java -version
+
+      - name: Install npm dependencies
+        run: npm ci
+
+      - name: Install bundler dependencies
+        run: bundle install --gemfile docs/Gemfile --deployment --jobs=4 --retry=3 --clean
+
+      - name: Build docs
+        run: npm run docs-compile
+
+      - name: Run HTML validator
+        run: npm run docs-lint