[GPU-Plugin] Major refactor 2 (#2664)

* Change cmake option

* Move source files

* Move google tests

* Move python tests

* Move benchmarks

* Move documentation

* Remove makefile support

* Fix test run

* Move GPU tests

doc/how_to/index.md

@@ -8,6 +8,7 @@ This page contains guidelines to use and develop XGBoost.
 ## Use XGBoost in Specific Ways
 - [Parameter tuning guide](param_tuning.md)
 - [Use out of core computation for large dataset](external_memory.md)
+- [Use XGBoost GPU algorithms](../gpu/index.md)
 
 ## Develop and Hack XGBoost
 - [Contribute to XGBoost](contribute.md)