Make tutorial a valid asciidoc article.

There should be only one level 0 title, so redefine underline rules in
asciidoc.conf to keep the existing underlines when possible.  Form a new
level 1 part "Technical Information" from the text at the end.  Make
introduction a preamble - it's too short to be a level 1 section.

Add doc/Makefile to process the tutorial automatically.  make needs to
be run explicitly in the doc directory.

Signed-off-by: Pavel Roskin <proski@gnu.org>