A number of improvements to the SQL cache store docs based on feedback:

• Add link to the persistence code tutorial.
• Clarify details about data types and Protobuf schemas. DG automatically selects the type if the keys and values map to single database columns. You need to provide Protobuf schema only if keys/values are composites of multiple columns.
• For SQL table stores, update the schema part of the cache configuration to show "books" as in the Protobuf examples for composite keys/values.
• For SQL query stores, add SQL DDL statements, corresponding Protobuf schemas, and matching schema configuration for the caches.
• Add details about generating Protobuf schema with the ProtoStream processor.