docs: Update Doxyfiles for c-doc

docs: Update Doxyfiles for c-doc

- Only the cautilinterface.h from connectivity is currently
  intended as a public API. The documentation directly
  references values defined in cacommon.h. So cacommon.h
  is also included to prevent documentation errors.
- OPTIMIZE_OUTPUT_FOR_C has been changed to yes. This prevents
  the documentation adding things about classes and other
  documentation that does not make since in C only code.
- Updated the Home.txt. The page linked to no longer contains
  getting started guides.

Change-Id: I1bb5d42fcd1287bb0f5614ceb204b1e56f79aa2d
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18137
Reviewed-by: Mats Wichmann <mats@linux.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>