Clean up "source" strucure for Vitess connector

There are few loose ends around the "source" block of the Vitess connector.

• Show the new "keyspace" attribute in examples in the connector docs
• "schema" attribute: seems like it's not needed if there's no such concept in Vitess? If so, let's simply remove it, no need for a deprecation cycle as this connector is incubating
• db: currently set to logical name database.server.name; the logical name is already conveyed via the "name" attribute"; if there's no notion of logical databases in Vitess, let's set it to an empty string (unfortunately, the field is marked as non-optional in the source parent schema shared between all connectors)