Document some advanced use-cased for exportimport

[pbauer]

Document best practices and tips:

• Migrate to Volto (How to migrate to Volto using exportimport #133)
• Defer imports (e.g. when field values are from a vocabulary based on content that might not yet exists)
• Export/Import registry settings (Add export and import for non-default registry settings #130)
• Export/Import Annotations
• Export/Import Marker Interfaces
• Export/Import Zope Users
• Export/Import portal properties settings
• Export/Import installed Add-ons
• Export PloneFormGen as Easyform
• Alternative ways to handle items without parent
• Fix collection queries