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",

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]
+

docs/reference/gio/meson.build

@@ -232,6 +232,7 @@ expand_content_files = [
   'dbus-utils.md',
   'error.md',
   'file-attributes.md',
+  'io-scheduler.md',
   'migrating-gconf.md',
   'migrating-gdbus.md',
   'migrating-gnome-vfs.md',