Make document-level annotations searchable

[engisalor]

Is your feature request related to a problem? Please describe.
Document-level annotations are not indexed/searchable, unlike other annotation types. For instance, in a project with a Sentence Label layer, the CQL search <Sentence_Label/> will find this type of annotation. In contrast, <Document_Label/> returns nothing for projects that use a Document Label layer.

Describe the solution you'd like
Users may need to find a document based on its metadata, either in the Annotation interface or in the Documents interface. CQL searches that can make use of document-level data, e.g., <Document_Label/>, would help users and admin find sets of related documents.

1. The direct solution would be to index this annotation type just like the others.
2. A related step would be to add CQL search functionality in the Documents interface. Currently there's a "Filter by name" search bar, which lets admin select and delete sets of documents, but CQL searching (if only for document-level annotations) would be more powerful.

Describe alternatives you've considered

1. For the Annotation interface, document-level annotations can be added to an arbitrary token as a span-based layer, e.g., on the first token of every document. This can be a perfectly workable solution.
2. For the Documents interface, if admin need to filter documents by document-level metadata, it can be included in the filename and searched via "Filter by name". This works but can lead to rather long filenames and duplicates document-level metadata (requiring one set in an annotation and another in the filename).

Additional context
Improving these features for document-level annotations would help integrate this annotation type into Inception, although (2) probably warrants a separate issue if pursued.