docs: Add guide for migrating a self-hosted database to Cloud

[Zfinix]

Closes: serverpod/serverpod#5768
Closes: serverpod/serverpod#5769

Adds a guide for moving a self-hosted database (Serverpod and PostgreSQL in Docker, for example on a VPS) into a Cloud project. Nothing covers this today, and the obvious pg_dump and pg_restore --data-only route fails on the tables Cloud writes itself.

• The dump is taken with --data-only at the source, which orders tables by foreign key. A full dump restored with --data-only fails on serverpod_auth_core_profile.
• --exclude-table-data covers the data Cloud writes on each deploy, each pattern keeping a trailing * so the table's ID sequence is left out too.
• emailSecretHashPepper and the JWT keys are copied before any user rows land, otherwise sign-in fails with invalidCredentials and the first refresh deletes the session.
• Profile images take a two-pass restore, since serverpod_auth_core_profile and serverpod_auth_core_profile_image reference each other and the Cloud database user cannot disable triggers.
• Four troubleshooting entries for the errors a missed step produces.

Every step run against a Cloud project on the starter plan, with a Docker Compose source on PostgreSQL 16, at Serverpod 4.0.0 and Cloud CLI 1.0.0.

[Isakdl]

Very cool to get this in, I got some suggestions here and there.

[developerjamiu]

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