Adjust guidance based on Informatica expert call - INPLATFORM-115

After validating our shared org vs sub org document with Informatica, I made some edits to reflect their guidance. Overall, our original guidance lined up well with their recommendations. The main change was to recommend using a new parent org with sub-orgs instead of our original intention of creating sub-orgs under our shared org. I think this extra layer of isolation is good. There isn't a need to change ODMAS to this new way, but this is something we'll want to consider for future users who don't want to use the shared org.

cc @jont @NUWAN.KUMARASIRI @SHREYASH.SHRESTHA could your review?