Documentation: xfs: consolidate XFS docs into its own subdirectory

XFS docs are currently in upper-level Documentation/filesystems.
Although these are currently 4 docs, they are already outstanding as
a group and can be moved to its own subdirectory.

Consolidate them into Documentation/filesystems/xfs/.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>