docs: build demo workflows

[jitingxu1]

~We are currently targeting the NVTabular demo on RecSys2020 Challenge as a demo workflow.~ Update: Due to the RecSys2020 demo data being unavailable (and against Twitter's terms to share), we will start with the R nycflights13 dataset. It has been added to Ibis examples to support this.

Major tasks

1. Demo Dataset
   • nycflights13
2. Feature engineering
   • Use ibis and ibisml for data preprocessing
3. Model training
   • XGBoost
   • sklearn
   • PyTorch