sql/substr.md: new page

Summary

• sql/substr.md: new page

Address changes from picodata@25264bc3, sbroad@753b482e

Changes added to Picodata in picodata!1185 (merged)

picodata 24.5.0-313-g49823d24

ANSI: E021-06

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• Close #333 (closed)
• Cherry-pick to: 24.5
• Staging:
  • ../reference/sql/substr/
  • ../reference/ansi_sql/#mandatory_features
  • ../sql_index/
• Follow-up for picodata!1195 (merged), sbroad!501 (merged)
• See also:
  • https://postgrespro.ru/docs/postgrespro/16/functions-string