Add the ability to skip the numbering for a paragraph

Maybe two notions must be distinguished:

• hidden numbering: in this case a number is still assigned to the paragraph, but is simply not displayed (in this case, the sequence would go 1 _ 3)
• skipped numbering: in this case no number is assigned to the paragraphs (hence not displayed) (in this case, the sequence would go 1 _ 2)