doc: do not generate unused doxygen HTML and Latex output by default

The doc build needs not the HTML or Latex documentation generated by
doxygen, as sphinx takes only the XML. Disable them to use less space
and speed up the build.

Change-Id: I51974449262fc5b45c6c2b41aad54365cceac341
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>