sql/to_char.md: new page

Summary

• sql/to_char.md: new page

Address changes from picodata!963 (merged)

Version: Picodata 24.3.0-86-g728f5622 (picodata@728f5622)

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• Close #242 (closed)
• Cherry-pick to: 24.4
• Staging:
  • https://docs.binary.picodata.io/picodata/asabirov_add_to_char_function/reference/sql/to_char/
  • https://docs.binary.picodata.io/picodata/asabirov_add_to_char_function/reference/ansi_sql/#additional_features
  • https://docs.binary.picodata.io/picodata/asabirov_add_to_char_function/sql_index/