planner: add tidb_enable_binding_usage

[hawkingrei]

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[gemini-code-assist]

Summary of Changes

Hello @hawkingrei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces the documentation for a new system variable, tidb_enable_binding_usage, which allows users to manage the collection of usage statistics for SQL plan bindings within the TiDB system. This addition provides clarity on how binding usage information is recorded and controlled, enhancing the system's observability and configurability regarding query optimization.

Highlights

• New System Variable Documentation: Documentation for a new system variable, tidb_enable_binding_usage, has been added to the system-variables.md file.
• Purpose of tidb_enable_binding_usage: This new variable controls whether the usage statistics collection for SQL plan bindings is enabled. When active, bindinfo usage is written to TiKV every six hours.
• Variable Properties: The tidb_enable_binding_usage variable is a Boolean type, has a GLOBAL scope, persists to the cluster, and defaults to ON. It is introduced in v9.0.0.

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[gemini-code-assist]

Code Review

This pull request adds documentation for the new system variable tidb_enable_binding_usage. The documentation is clear and follows the established format. I have one minor suggestion to rephrase the description for better clarity and to align with the style guide's preference for active voice and present tense.