docs: Move the GIOScheduler SECTION

Move it to a separate page, as there isn’t a `GIOScheduler` struct.
Ensure that all its functions/methods/types are correctly marked as
deprecated. Fix a few broken links about I/O priority which pointed to
it.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/gio/gio.toml.in

@@ -55,6 +55,8 @@ content_files = [
   "migrating-gdbus.md",
   "migrating-gconf.md",
   "migrating-gnome-vfs.md",
+
+  "io-scheduler.md",
 ]
 content_images = [
   "gvfs-overview.png",