docs: renumber headings

docs: renumber headings

It's generally frowned upon to have multiple H1 headings in HTML4. So
let's make sure each article has a primary heading for the article, and
that that heading is the title that is used in the sidebar.

While we're at it, let's update the title in the articles to match the
title from the sidebar as well.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>