docs: Document RAG storage drivers, Qdrant attach, and sandbox wiring
Covers the pluggable RAG driver work: the yaml/duckdb/qdrant drivers and how
to choose between them, attaching a read-only remote Qdrant collection with
.rag attach, the driver field on graph rag nodes, and how an attached RAG's
host and API key reach a sandbox.
Also corrects two statements the driver work invalidated: embeddings are no
longer necessarily in-memory, and .rebuild rag now re-embeds the whole corpus
rather than just accommodating document changes.