Add SQL structure for customers, countries, and user preferences tables

Refactor README and API docs for clarity and consistency

Simplified the schema definition in the README and updated query examples to use more intuitive CQL classes. Updated data type definitions and column sizes for better precision. Replaced classes with structs in the API docs and corrected formatting issues for better readability. These changes enhance comprehension and maintain consistency across documentation.

Update README.md

Update module-cql-relations-manytomany.md

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update docs

update docs