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
version.

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.