Update RAG docs with vector db examples

[vga91]

Update docs (and add some tests) with something like the below one, i.e. to add examples to interface with vector db, when the issue #3971 will be merged

CALL apoc.vectordb.qdrant.query($host, $collection, [0.2, 0.1, 0.9, 0.7], {}, 5, $conf) YIELD node 
collect(node) as paths
WITH paths
CALL apoc.ml.rag(paths, [], $question, $conf) YIELD value
RETURN value
...