doc: update make clean to remove doxygen folder

Gerrit patch https://gerrit.zephyrproject.org/r/#/c/10849/


created a doxygen folder that make clean should remove

Change-Id: I5cd4dea489c868eef4f101c20cced3280b7abc39
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>