chore(examples): migrates `draft-preview` example to 3.0

[PatrikKozak]

What?

Migrates the draft-preview example to payload 3.0.

Updates:

• draft-preview/payload now has a next app directly along side payload - new recommended approach (i.e admin panel & front-end on the same port - 3000)

• Keeps draft-preview/next-app & draft-preview/next-pages to demo running payload & your next app separately (i.e admin panel on port 3000 & front-end on port 3001)

To run the next-app or next-pages example - simply boot up the payload directory and then boot up next-app or next-pages