ext4: import on-disk layout book from wiki page

Create the basic structure of the "new" data structures & algorithms
book to be ported over from the on-disk format wiki, and then start by
pulling in the introductory information.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>