LU-5147 doc: design docs in documentation dir

Move design documents into the ./Documentation directory.
Update references to design documentation in the source code.
Minor readability updates to ldiskfs.txt.

Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Change-Id: Ia4d1662225d019358876caade6f564c48f450fff
Reviewed-on: http://review.whamcloud.com/10618
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Christopher J. Morrone <morrone2@llnl.gov>