doc: move documetnation context to root directory

This will allow inclusion of documentation files that exist outside of
doc/ and will make it possible to add rst files across the tree,
especially for boards, samples and tests.

Change-Id: I7afcf92d99f504b2bc0b2b7e3452acb2f8e08294
Signed-off-by: Anas Nashif <anas.nashif@intel.com>