Add file-based document store and vector store (#96)

* Modify docstore and vectorstore objects to be reconstructable
* Simplify the file docstore
* Use the simple file docstore and vector store in MVP

knowledgehub/storages/vectorstores/__init__.py

@@ -1,5 +1,11 @@
 from .base import BaseVectorStore
 from .chroma import ChromaVectorStore
 from .in_memory import InMemoryVectorStore
+from .simple_file import SimpleFileVectorStore
 
-__all__ = ["BaseVectorStore", "ChromaVectorStore", "InMemoryVectorStore"]
+__all__ = [
+    "BaseVectorStore",
+    "ChromaVectorStore",
+    "InMemoryVectorStore",
+    "SimpleFileVectorStore",
+]