Add documentation about jsonb_accessor gem

In !123380 (merged), we added a gem jsonb_accessors that allows use to ActiveRecord validations. This is an addition to JsonSchemaValidator

We introduced this because the ApplicationSettings and OrganizationSettings have complex validations:

• Custom error messages for UI purposes
• Validations against values defined in code
• Validations using methods

The documentation for storing JSON in the database at https://docs.gitlab.com/ee/development/migration_style_guide.html#storing-json-in-database should be updated:

• Add this gem as an alternative
• Short code example
• Guidelines: when should a developer use JsonSchemaValidator or jsonb_accessors