Document clean removal of a FDW

[Oliver Rice]

Improve documentation

Moving feedback from notion (team only link) https://www.notion.so/supabase/team-postgres-16cf7874025b4b5daf89797af70ce739?p=6543dddb62b84dcfa770386efbf22528&pm=s

Was trying to clean my project from old foreign data wrappers, in order to make a clean migration script.

I wasn’t sure how exactly I can remove everything cleanly. Would be nice to have a guide on that.