QTQA: Added make html_docs step

CI needs ability to run make html_docs inside defined stage. Added
make step and property handling to ignore failures if needed.

Change-Id: I87d11184960289716c6614709254457352d0e41e
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>