docs: add statement summary system variables

[nolouch]

What is changed

Add English documentation for two statement summary system variables, corresponding to pingcap/docs-cn#21694:

• tidb_stmt_summary_group_by_user
• tidb_stmt_summary_persist_evicted

Updated files:

• statement-summary-tables.md
• system-variables.md
• system-variable-reference.md

Check List

Tests:

• ./scripts/markdownlint statement-summary-tables.md system-variable-reference.md system-variables.md
• git diff --check -- statement-summary-tables.md system-variable-reference.md system-variables.md

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[gemini-code-assist]

Code Review

This pull request adds documentation for two new system variables introduced in TiDB v8.5.7: tidb_stmt_summary_group_by_user and tidb_stmt_summary_persist_evicted. The changes span across statement-summary-tables.md, system-variable-reference.md, and system-variables.md. The review comments suggest rewriting several sentences to use the active voice, reducing wordiness, and using "who" instead of "that" when referring to users, in alignment with the repository's documentation style guide.

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