docs: give download-article a primary heading

docs: give download-article a primary heading

It's generally frowned upon to have multiple H1 headings in HTML4. So
let's add a primary heading for the article, and source that from the
title used in the sidebar.

While we're at it, let's update the title in the article 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>