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

src/libostree/ostree-sysroot-private.h

@@ -34,6 +34,10 @@ typedef enum {
 
 } OstreeSysrootDebugFlags;
 
+/**
+ * OstreeSysroot:
+ * Internal struct
+ */
 struct OstreeSysroot {
   GObject parent;