qdoc: Prepare QDoc for the new style on qt.io

The new template and CSS have some requirements that need changes
in the generated .html:

    - Generate a new div 'sidebar' and place the TOC (if one
      exists) inside it, allowing the template to extend the
      sidebar contents dynamically. Do this for all pages
      except index.html.

    - Change the DOCTYPE declaration to be html5-compliant

    - Replace <tt> tags with <code> to be html5-compliant

    - Add a new config variable HTML.prologue - this allows
      the template to insert custom html into beginning of
      the page, before the page title but after any navigation
      or table-of-contents items.

    - Wrap tables inside <div> elements. This allows for
      better-working CSS design for small-screen devices.

    - Write out extra parameters first when outputting
      function synopsis to have better styling.

    - Inject zero-width-space characters into function
      names to allow the browser break up long function
      signatures in a nice manner.

    - Edit the CSS for the offline style to adapt to
      above changes.

Task-number: QTBUG-42086
Change-Id: I3075cdc11bcb07a66150388519263fd721c8002b
Reviewed-by: Martin Smith <martin.smith@digia.com>