Support link features in document-level annotations in auto-curation

[reckart]

Is your feature request related to a problem? Please describe. Link-feature auto-merging is currently only supported for span features. However, document-level layers also support adding link features.

Describe the solution you'd like Allow auto-merging document-level link features as well.

Describe alternatives you've considered Demote document-level link features to experimental status.