documentation: Style sheet naming changes

I had to rename individual style.css sheets to have book-specific
names.  So style.css for the ADT manual becomes adt-style.css and
so forth.  Turns out, this isn't really necessary for the solution
to create the mega-manual but I am leaving the changes as a
separate commit because it makes sense.  It causes no disruption
to making the six YP manuals.

(From yocto-docs rev: 8cbdae1fe86988243f91dfd611c14f8a37007886)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>