add ft_one_hot_encoder to docs and move data frame functions above ml functions
add ft_one_hot_encoder to docs and move data frame functions above ml functions

kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr