(docs) MMR page switch to AstraDBVectorStore (=data api)

[hemidactylus]

The "VectorStore QA with MMR" page is showing usage of the Cassandra vector store class.

This PR switches the page to using the AstraDBVectorStore, i.e. the Data API connector, keeping everything else as it was.
In particular:

• change in the env vars (endpoint in place of db id)
• remove note on how to extract db id from endpoint
• change in the var name for the v.store variable and how it is initialized
• changes reflected to the separate mmr-example.adoc file.

NOTE the "open in colab" link at the top is still pointing to the Cassandra version of the same workload ... because the code I'm contributing is not hosted as a colab-friendly notebook anywhere. I can add it if desired, but then I'd suggest the location to be a colab/notebook dedicated directory inside this repo (easier maintenance). LMK.

[cbornet]

LGTM.
NIT: lowercase variable names if possible.