Documentation: create new scheduler/ subdirectory

The top-level Documentation/ directory is unmanageably large, so we
should take any obvious opportunities to move stuff into subdirectories.
These sched-*.txt files seem an obvious easy case.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>