docs: drop h1 in header

docs: drop h1 in header

It's generally frowned upon to have more than one H1 per document in
HTML4. So let's put the text directly inside the header. This means we
can drop the flex-based centering, which makes things a bit easier. We
also need to change the padding to rem instead of em, because the em has
now changed.

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