Decrement level of all titles, except main title

The rationale is to keep the level 0 for the title of the document,
and start other titles at level 1.

The example in section 11.2 One line titles shows that level 0
is the document title, while levels 1 to 4 correspond to sections
of the document.

Reference:
http://asciidoc.org/userguide.html#X17