xfs: document the filesystem metadata checking strategy

Begin the fifth chapter of the online fsck design documentation, where
we discuss the details of the data structures and algorithms used by the
kernel to examine filesystem metadata and cross-reference it around the
filesystem.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Documentation/filesystems/xfs-self-describing-metadata.rst

 .. SPDX-License-Identifier: GPL-2.0
+.. _xfs_self_describing_metadata:
 
 ============================
 XFS Self Describing Metadata