message: docs: use proper sections in aio home
  convert the `div` groups in the aio home page
  to proper `section`s with a valid heading/title
  PR Close
diff: </div>
  <div class="text-container">
  - <div class="text-block promo-1-desc l-pad-top-2">
  - <div class="text-headline">Develop Across All Platforms</div>
  + <section aria-labelledby="promo-1-title" class="text-block promo-1-desc l-pad-top-2">
  + <h2 id="promo-1-title" class="text-headline no-anchor">Develop Across All Platforms</h2>
  <p class="text-body">Learn one way to build applications with Angular and reuse your code and abilities to build apps for any deployment target. For web, mobile web, native mobile and native desktop.
  </p>
  - </div>
  + </section>
  </div>
  </div>
  <hr>
  <!-- Group 2 -->
  <div layout="row" layout-xs="column" class="home-row">
  <div class="text-container">
  - <div class="text-block">
  - <div class="text-headline">Speed & Performance</div>
  + <section aria-labelledby="promo-2-title" class="text-block">
  + <h2 id="promo-2-title" class="text-headline no-anchor">Speed & Performance</h2>
  <p class="text-body">Achieve the maximum speed possible on the Web Platform today, and take it further, via Web Workers and server-side rendering.</p>
  <p class="text-body">Angular puts you in control over scalability. Meet huge data requirements by building data models on RxJS, Immutable.js or another push-model.</p>
  - </div>
  + </section>
  </div>
  <div class="promo-img-container promo-2">
  </div>
  <div class="text-container">
  - <div class="text-block promo-3-desc">
  - <div class="text-headline">Incredible Tooling</div>
  + <section aria-labelledby="promo-3-title" class="text-block promo-3-desc">
  + <h2 id="promo-3-title" class="text-headline no-anchor">Incredible Tooling</h2>
  <p class="text-body">Build features quickly with simple, declarative templates. Extend the template language with your own components and use a wide array of existing components. Get immediate Angular-specific help and feedback with nearly every IDE and editor. All this comes together so you can focus on building amazing apps rather than trying to make the code work.
  </p>
  - </div>
  + </section>
  </div>
  </div>
  <!-- Group 4 -->
  <div layout="row" layout-xs="column" class="home-row">
  <div class="text-container">
  - <div class="text-block l-pad-top-2">
  - <div class="text-headline">Loved by Millions</div>
  + <section aria-labelledby="promo-4-title" class="text-block l-pad-top-2">
  + <h2 id="promo-4-title" class="text-headline no-anchor">Loved by Millions</h2>
  <p class="text-body">From prototype through global deployment, Angular delivers the productivity and scalable infrastructure that supports Google's largest applications.</p>
  - </div>
  + </section>
  </div>
  <div class="promo-img-container promo-4">
  