docs: make website launch-ready

[deepyaman]

https://ibis-project.github.io/ibis-ml/

• [P0] Installation
• [P0] Have a getting started example in the documentation (nycflights13 demo)
• [P0] Why IbisML?
• [P1] Concepts
• [P1] How-to guides (e.g. how to write your own Step, how to use IbisML without scikit-learn, inverse transform (when available?), how to use arbitrary Ibis expressions, etc.?) (see docs(website): create skeleton for new FAQ section #92)