update Makefile with new layout

I've changed the layout of this repo to more closely mirror
https://github.com/riscv/docs-spec-template



Since the working adoc files are located in a subfolder, I've updated
the Makefile to point at that "root file". An "header.adoc" should be
added that more closely matches the template header.

Signed-off-by: Stephano Cetola <scetola@linuxfoundation.org>