docs: update nightly docs for information_schema.statistics

[Copilot]

Closes #2558
Closes #2534

What's Changed in this PR

This updates the nightly English and Chinese docs for the new information_schema.statistics table introduced by GreptimeDB, and aligns nearby INFORMATION_SCHEMA references with the new index metadata behavior.

• New STATISTICS reference page

  • Add nightly STATISTICS docs in en and zh.
  • Document schema, MySQL-compatible semantics, SHOW INDEX relationship, and example output for primary, time, inverted, fulltext, and skipping indexes.

• Overview and navigation updates

  • Replace STATISTICS = “Not supported” in INFORMATION_SCHEMA overview with the new doc link.
  • Add the nightly page to sidebars.ts.

• Related doc corrections from the same feature

  • Clarify that KEY_COLUMN_USAGE now covers key constraints, while secondary index metadata lives in STATISTICS.
  • Update TABLE_CONSTRAINTS wording to point readers to STATISTICS / SHOW INDEX.
  • Normalize COLUMNS.is_nullable examples to YES / NO to match current metadata output.

SELECT table_schema,
       table_name,
       non_unique,
       index_name,
       seq_in_index,
       column_name,
       index_type,
       greptime_index_type
FROM information_schema.statistics
WHERE table_schema = 'public'
  AND table_name IN ('system_metrics', 'test')
ORDER BY table_name, index_name, seq_in_index, column_name;

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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@fengjiachun PTAL