doc: modify style for texi2html 1.78+
The generated HTML files are similar to the ones generated with
texi2html 1.56k used on the website.
Tested with texi2html 1.78 and 5.0. 1.78 is the minimal recommended
The removed @sp from the titlepage section were ignored until
texi2html 5.0. If not removed the pages generated by 5.0 will have ugly
empty space around the title.