Add section on autocorrelation and HAC standard errors

Expanded the TF-ITS notebook with a detailed explanation of autocorrelation in time series, its impact on causal inference, and the motivation for using HAC (Newey-West) standard errors. Updated the simulation to generate autocorrelated errors using an AR(1) process, and clarified the importance of robust inference in the context of time series interventions.

Author: drbenvincent