Document how to add relations to MongoDB after introspection (Epic 2)

[matthewmueller]

MongoDB doesn't support automatically adding relations after introspection, so you need to do it by hand. We should document how to do this.

[matthewmueller]

Notes with Docs:

• Available in getting started: https://www.prisma.io/docs/getting-started/setup-prisma/add-to-existing-project/mongodb/introspection-typescript-mongodb#tweaking-the-schema
• Don't seem to have a regular place, but there's a slack thread: https://prisma-company.slack.com/archives/C013RMLGE72/p1647951893323199

[matthewmueller]

Closed by: #3017