Clarify Astra DB keyspace limitation

[aimurphy]

Hello - I am a technical writer for DataStax.

This PR clarifies the limitation on CREATE KEYSPACE for DataStax Astra DB. This restriction only applies to Astra DB, not DSE or HCD. Additionally, you can use the Astra Portal, DevOps API, or Astra CLI to create a keyspace. I have updated the text, and I added links to more relevant pages.

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