Implement a Document Index backed by Weaviate.

References:

• Implementation manual: https://github.com/docarray/docarray/blob/feat-rewrite-v2/docs/tutorials/add_doc_index.md
• Document Index design doc (slightly outdated, but the main ideas remain): https://lightning-scent-57a.notion.site/Document-Stores-v2-design-doc-f11d6fe6ecee43f49ef88e0f1bf80b7f
• Original PR adding Document Index base class: feat: hnswlib document index #1124