[gtk-doc] Initial setup

Initial setup of gtk-doc. Straight forward setup following the gtk-doc
instructions. Ignore some troublesome types in src/hb-gobject.h. To
build use "./autogen.sh --enable-gtk-doc" then "make". Docs are in
harfbuzz/docs/reference/html/index.html.

src/Makefile.am

diff --git a/src/Makefile.am b/src/Makefile.am
index 73804a1..e646d4c 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -293,7 +293,7 @@
 	hb-ot-shape-complex-myanmar-machine.rl \
 	hb-ot-shape-complex-sea-machine.rl \
 	$(NULL)
-%.hh: %.rl
+.rl.hh:
 	$(AM_V_GEN)$(top_srcdir)/missing --run ragel -e -F1 -o "$@.tmp" "$<" && \
 	mv "$@.tmp" "$@" || ( $(RM) "$@.tmp" && false )