Replace the outline algorithm with one based on heading levels

Removes the idea of sectioning content influencing the document's outline. Instead, the outline is derived from all of the document's h1-h6 elements. (Sectioning content still plays a role in scoping header and footer elements.) This ensures that the outline generated by the outline algorithm, i.e. the document's semantics for authors, better aligns with the way screen reader users navigate through headings and the corresponding normative requirements for implementations in the HTML-AAM specification.

Updates the hgroup element to have a new content model, to use p elements for related content such as subheadings, alternative titles, or taglines (instead of using heading elements for those).

Updates all of the spec's examples to have proper outlines, according to the new outline algorithm. This includes preferring h2 to h1 in markup fragment cases, so as to better suggest that they are part of a larger document whose outline contains a h1.

Closes #83. Closes #3499 by superseding it. Closes #6462 by updating hgroup's suggested usage instead of obsoleting it. See #7867 for potential followup work in avoiding sectioning content and hgroup influencing the style of h1-h6 elements.

Co-authored-by: Anne van Kesteren <[email protected]>
Co-authored-by: Domenic Denicola <[email protected]>