Add DMatrix usage examples to c-api-demo (#5854)
Add DMatrix usage examples to c-api-demo (#5854)

* Add DMatrix usage examples to c-api-demo

* Add XGDMatrixCreateFromCSREx example

* Add XGDMatrixCreateFromCSCEx example
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost