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/io-scheduler.md

@@ -0,0 +1,20 @@
+Title: GIOScheduler
+SPDX-License-Identifier: LGPL-2.1-or-later
+SPDX-FileCopyrightText: 2007 Andrew Walton
+
+# GIOScheduler
+
+Schedules asynchronous I/O operations. `GIOScheduler` integrates
+into the main event loop ([struct@GLib.MainLoop]) and uses threads.
+
+Deprecated: 2.36: As of GLib 2.36, `GIOScheduler` is deprecated in favor of
+[struct@GLib.ThreadPool] and [class@Gio.Task].
+
+The `GIOScheduler` API is:
+ * [type@Gio.IOSchedulerJobFunc]
+ * [func@Gio.io_scheduler_push_job]
+ * [func@Gio.io_scheduler_cancel_all_jobs]
+ * [type@Gio.IOSchedulerJob]
+ * [method@Gio.IOSchedulerJob.send_to_mainloop]
+ * [method@Gio.IOSchedulerJob.send_to_mainloop_async]
+