[AUR-420] Provide document store base interface and an in-memory version (#21)

Document store handles storing and indexing Documents. It supports the following interfaces:

- add: add 1 or more documents into document store
- get: get a list of documents
- get_all: get all documents in a document store
- delete: delete 1 or more document
- save: persist a document store into disk
- load: load a document store from disk

knowledgehub/docstores/__init__.py

@@ -0,0 +1,4 @@
+from .base import BaseDocumentStore
+from .simple import InMemoryDocumentStore
+
+__all__ = ["BaseDocumentStore", "InMemoryDocumentStore"]