ext4: import superblocks chapter from wiki page

Import the chapter about superblocks from the on-disk format wiki
page into the kernel documentation.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Documentation/filesystems/ext4/ondisk/globals.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+Global Structures
+=================
+
+The filesystem is sharded into a number of block groups, each of which
+have static metadata at fixed locations.
+
+.. include:: super.rst

Documentation/filesystems/ext4/ondisk/index.rst

@@ -5,3 +5,4 @@ Data Structures and Algorithms
 ==============================
 .. include:: about.rst
 .. include:: overview.rst
+.. include:: globals.rst