SubDoc lookupIn Docs authoring

For the SDK 3.0 documentation, author the section on lookupIn type SubDocument Operations.

GDrive link for early editors who are passionate about the material: https://docs.google.com/document/d/1M8JKNSA3Uc3wBhyig_UtIuliNB0Bq2CEWSegsGj9TRQ/edit

Given that it's pathfound but we want some more review, goal is to get review by EOD April 17, then others start on their authoring after that. Richard Smedley will put the .adoc templates in on April 18, when others can start authoring.