sql/coalesce.md: new page

Summary

• sql/coalesce.md: new page

Address changes from picodata@291323c2

24.7.0-67-g291323c2

ANSI: F261-04

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• Close #400 (closed)
• Staging:
  • ~/reference/sql/coalesce/
  • ~/sql_index/
  • ~/reference/ansi_sql/