docs: Move the gunixmounts SECTION

Move it to a separate page.

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

Helps: #3037

docs/reference/gio/unix-mounts.md

@@ -0,0 +1,35 @@
+Title: UNIX Mounts
+SPDX-License-Identifier: LGPL-2.1-or-later
+SPDX-FileCopyrightText: 2007 Andrew Walton
+SPDX-FileCopyrightText: 2008 Matthias Clasen
+
+# UNIX Mounts
+
+Routines for managing mounted UNIX mount points and paths.
+
+Note that `<gio/gunixmounts.h>` belongs to the UNIX-specific GIO
+interfaces, thus you have to use the `gio-unix-2.0.pc` pkg-config
+file when using it.
+
+There are three main classes:
+
+ * [struct@Gio.UnixMountEntry]
+ * [struct@Gio.UnixMountPoint]
+ * [class@Gio.UnixMountMonitor]
+
+Various helper functions for querying mounts:
+
+ * [func@Gio.unix_mount_points_get]
+ * [func@Gio.UnixMountPoint.at]
+ * [func@Gio.unix_mounts_get]
+ * [func@Gio.unix_mount_at]
+ * [func@Gio.unix_mount_for]
+ * [func@Gio.unix_mounts_changed_since]
+ * [func@Gio.unix_mount_points_changed_since]
+
+And several helper functions for checking the type of a mount or path:
+
+ * [func@Gio.unix_is_mount_path_system_internal]
+ * [func@Gio.unix_is_system_fs_type]
+ * [func@Gio.unix_is_system_device_path]
+