fix(ET-1265): Updates migration service documentation

[ksnavely]

What Changed?

I've updated some documentation content based on testing and review.

Not In Scope

Anything unrelated to MP/MTK schema management. I made some small updates but am stopping to avoid scope creep.

[gvasquezvargas]

Hi all! There is some overlap with https://github.com/EnterpriseDB/docs/pull/5883/files. Specifically regarding the handling of the database schema. Let's have a sync and decide how to best walk the customer through the preparation of database schemas.

[gvasquezvargas]

Closing in favor of https://github.com/EnterpriseDB/docs/pull/5922