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

[deepyaman]

Initial version merged in #60. Some remaining TODOs:

• [ ] Make sure unique key is actually unique
• [ ] Do we want to do something with weather data? We join it, and then throw away all the columns...
• [ ] Include TargetEncoding step
• [ ] Clearly document handoff to other modeling frameworks (did test XGBClassifier works, but add it to a notebook; also, make sure PyTorch demo is there, maybe something like skorch with MLP)... (maybe add model choice to demo notebook)
• [ ] Once Ibis 9.0 is released, update demo notebook to pull directly from ibis.examples.nycflights13_*.fetch() instead of local DuckDB