LangChain, QA Basic notebook improvements

[hemidactylus]

(some of the points below will propagate to all notebooks)

Title => Vector Similarity Search QA Example

Keep it terse: Astra only until Cassandra ships VSS as well.

~Make steps more straightforward~ Moved to a separate issue

Subtitle and explanation on the "spawn a retriever" part.

Avoid confusion by renaming ASTRA_DB_CLIENT_ID to ASTRA_AUTH_TYPE (or a varation of username/password paired names, maybe even better)

A note on having to restart the runtime

[hemidactylus]

done, merged to main