Documentation Update on Instrument Variable Methods

[huigangchen]

Proposed changes

The changes cover the added documentation on instrument variable related methods as well as a description of TMLE algorithm. Specifically I added descriptions to DR-Learner, DR-RV Learner and as background information I included a short description on LATE and expanded the section on 2SLS. The change also includes TMLE for ATE and linked back to the validation method.

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[paullo0106]

LGTM except a minor typo :)

[paullo0106]

typo for "DR-Learner"

[huigangchen]

Thank you @paullo0106! Just corrected the typo.

[paullo0106]

thanks!