docs: add statement summary session variables

[nolouch]

What is changed, added or deleted? (Required)

Added documentation for two statement summary system variables:

• tidb_stmt_summary_group_by_user: controls whether statement summary rows are additionally grouped by executing user.
• tidb_stmt_summary_persist_evicted: controls whether evicted statement summary records are persisted to the statement summary log with an evicted marker.

Also updated the Statement Summary Tables page and the system variable reference index.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):
  • util/stmtsummary: add tidb_stmt_summary_group_by_user tidb#68512
  • util/stmtsummary: add tidb_stmt_summary_persist_evicted tidb#68513

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[nolouch]

ref pingcap/tidb#68512, pingcap/tidb#68513