docs: install doxygen HTML output into output directory

This will move autogeneated html files to the final directory
which is copied by CI into the online documentation making the
raw doxygen html files accessible via <url>/api/

Change-Id: I79bc211377840358acfb94d4de839ce25d2830e0
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
1 file changed