Added Gtk-Doc in 3.0

[Mark Doliner]

Thanks for doing this!

Do you know if it's possible to build the docs without compiling? (Not
critical, but would be nice for our nightly doc generation script that
lived at https://developer.pidgin.im/doxygen/)

Do you have plans for doc/PERL-HOWTO.dox and doc/TCL-HOWTO.dox? Maybe
these should be converted to gtk-doc?

"make dist" seems unhappy. Not exactly sure why. I don't have time to
look at it right now. If anyone else wants to take a stab at it, feel
free. The basic error is:
make[4]: Entering directory
`/home/dude/code/pidgin-main-clean/doc/reference/libpurple'
cp: cannot stat ‘./html/*’: No such file or directory