Use makeinfo to generate html doc for the new website

texi2html is deprecated by upstream in favor of makeinfo/texi2any. See:

- https://www.gnu.org/software/texinfo/manual/texinfo/html_node/texi2html.html
- https://wiki.debian.org/Texi2htmlTransition
- https://lists.debian.org/debian-devel/2013/05/msg01516.html



This is actually two separate changes.

Based on a patch by Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>.

Fixes Trac ticket #3232.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>