feature: sections rework

[davidsneighbour]

• move any <section> to the outermost block within the page structure. the only thing that can be outside of <section> can be a single div with a container class.

  <main>
  <div class="container" id="first-block">
    <section>
    ...
    </section>
  </div>
  <section>
  ...
  </section>
  </main>

  (that container can be used for spacing and positioning)
• any <section> must have a <header> with a headline tag with a proper counter

  <section>
    <header>
      <h2>Something</h2>
    </header>
    ...
  </section>

  use aria tags, visibility, and display classes to hide/show

[davidsneighbour]

seems to be done.