doc: added crate-level documentation comment

[sttk]

T/O.

[Copilot]

Pull Request Overview

This PR adds comprehensive crate-level documentation to explain the Redis data access functionality and transaction handling strategies within the Rust sabi framework. The documentation describes three key approaches for handling Redis transactions in mixed-database environments where traditional rollback mechanisms are not available.

Key changes:

• Added detailed crate-level documentation explaining DataSrc and DataConn derived classes
• Documented three transaction handling strategies: "force back", "pre-commit", and "post-commit"
• Provided guidance on when and how to use each strategy for Redis transaction management

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