Document migrations setup

[jbmoelker]

Should include:

• using environments
• creating / autogenerating migrations (could add a migrations:create script to help out)
• use maintenance mode (could use script npx datocms maintenance:on|off, see datocms docs)
• apply migrations (could use a migrations:apply script to help out)
• promoting environments (could use a migrations:promote script to help out, see promote)
• refer to this in docs/upgrading.md as performing a migration is required in case of new migration files.