New version adapted to nullable columns and null handling in multi-stage engine

[gortiz]

This is a follow up on https://github.com/pinot-contrib/pinot-docs/pull/299 that updates the null handling documentation to 1.1.0, which includes:

• Column level nullability
• Null handling in multi-stage engine

[gortiz]

Jackie already review this document, so we can merge it whenever we think it is ready.