Improve migrations docs: custom SQL, Serverpod Cloud apply guidance, troubleshooting

[developerjamiu]

Demand

Kapa "Applying/Running Migrations" + "Database Migrations" clusters — ~12 conversations combined.

Problem

06-database/11-migrations.md documents --apply-migrations, --role maintenance, and repair migrations, but:

• Custom SQL migrations are mentioned without structure, example, or when-to-use guidance.
• How migrations are applied on Serverpod Cloud (automatically or manually in the managed environment) is unclear.
• Common mistakes are not addressed: the --apply-migration (singular) error; "removing migrations" actually means rolling back via a repair migration, not deletion.

Scope

• A "Writing custom migrations" section (file structure, when/why, data migrations).
• Serverpod Cloud apply guidance.
• A "Troubleshooting" subsection.