Document entity removal

https://redmine.teklia.com/issues/11004

Requires backend#1922 (closed), closes #80 (closed)

• The Project → Entities page can be removed
• The Project → Database exports page should be updated to reflect the export database changes (export#13 (closed))
• The upgrades notes for Arkindex 1.9.0 should mention:
  • the entity removal
  • the long SQL data migration that rewrites the entire TranscriptionEntity table
  • what to do when there are duplicates that violate the unique_transcription_entity constraint
  • the need for reindex --drop to rebuild the Solr index