doc: Add support for generating HTML versions of the man pages.

* doc/Makefile.am (yat2m-stamp): Also call yat2m with --html options.
* doc/yat2m.c (main): Add dummy options.

Note that the generated html versions of the man pages will only be
correct if the external yat2m tool is installed - at least for the
maintainers of the website this will be the case.