docs: Get API docs working again

This changes around a few things that didn't work for me:
* Section names seem to be ostree-* instead of libostree-*
* Also XML files are ostree-* (they didn't show up at all)
- gtk-doc doesn't seem to parse const _OSTREE_PUBLIC correctly
* pull documentation is now on the actual functions rather than stubs
* Update gitignore with some more files

And there some changes to make gtk-doc give fewer warnings (not finished)

Closes: #327
Approved by: cgwalters

apidoc/.gitignore

@@ -32,6 +32,7 @@
 /ostree.args
 /ostree.hierarchy
 /ostree.interfaces
+/ostree-overrides.txt
 /ostree.pdf
 /ostree.prerequisites
 /ostree.signals
@@ -48,4 +49,5 @@
 /tmpl/*.bak
 /tmpl/ostree-unused.sgml
 /xml
+/version.xml
 _libs