ext4: import directory layout chapter from wiki page

Import the chapter about directory layout 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/dynamic.rst

@@ -8,3 +8,4 @@ allocated to files.
 
 .. include:: inodes.rst
 .. include:: ifork.rst
+.. include:: directory.rst