Update Import/Export development docs to mention which tests need updates when adding new relations

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This issue is the outcome from discussion in !32577 (comment 353955965)

When adding new relations to Import/Export it's not obvious that certain tests need to be added to tree saver/restorer, fixture updates and possibly other places.

Add a new section to https://docs.gitlab.com/ee/development/import_export.html specifying which tests and fixture need updates when adding new relations.