Add a summary for product-print-label

We have a lot of sections in that page (glabels file configuration, parameter
configuration, howto print), so add a summary at the beginning so it's easier
to find the information. Also transform the first part in a section so it's
automatically added to the summary too.

Change-Id: Ic0c93ac6166d37faa53bfa6efd9efa438d4e6cb5
Merge "Add a summary for product-print-label"