Add isLinkedDoc to elter.ftlh

Commit b4baebf6 introduced isLinkedDoc in _title.ftlh for rendering Gemini documents, presumably not realising that elter and ukems documents also rely on it. As a result, trying to open elter and ukems documents in the application has consistently resulted in 500 errors.

We fix only the elter documents here by assigning this property as always false, as another PR deals with ukems. We don't need this property as elter has its own concept of linked documents.

Closes EDS-3122