Add optional author line in header

The goal is to trigger the display of the title, currently missing
from github preview.
Merge branch 'i18n' into asciidoc

Conflicts:
	dataflow.asc
Replace two-line titles with one-line titles

This is an attempt to get titles rendered as titles in github preview,
rather than the start of a code block...
Remove ruler delimiters around introduction

Using one-line titles lead to a correct rendering of titles in github preview,
except for the top-level title. This is an attempt to fix this issue.
Revert "Remove ruler delimiters around introduction"

This reverts commit c26075fea7479a261cab3f3e921d3ecef8b64376.

This had no impact.
Replace top-level title with level 1 title, as a turn-around