Move the Devhelp API reference to devhelp2
The devhelp2 format has been introduced in 2005, and the old devhelp format was deprecated in 2017. Libxml2 is the last remaining project that still uses the old devhelp format. Since I'm planning to drop support for the old devhelp format in Devhelp 41 (released in September 2021), I think this would be a good time to switch to the devhelp2 format.
The main change between devhelp and devhelp2 formats is the replacement of the <function>
element with the <keyword>
one.
I'm happy to look into adding a devhelp2 index file.