[Doc] Add info on GPU compiler (#6204)
[Doc] Add info on GPU compiler (#6204)

* Add note about the required compiler version for CUDA. 
* Also added a link that gives a short explanation on compute capability version
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost