Remove h-tags from post-header

[gfellerph]

The current heading structure that is rendered with the header poses some SEO concerns and creates an HTML structure that is not exactly spec compliant. As the header is the first element on the page, hidden headings are rendered before the real <h1> of the page and are not starting at level 1.

Tasks

• Move hidden headings to appropriate aria descriptions
• Rework main page heading to be rendered in a <p> tag instead of a heading

[alizedebray]

• Investigate if it is possible to remove the heading tags on header elements that look like heading
• Adapt our docs to fit current Cargo implementation (using span instead of h1 as the header title for example)
• Modal suggestion