sql/cast: add syntax with `::` operator and edit

Summary

• cast.svg: add syntax with :: operator
• cast.md: proofread and edit

Address changes from picodata@b1c8a104

24.5.0-236-gb1c8a104

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• Close nothing
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/asabirov_update_cast/reference/sql/cast/
• Follow-up for picodata!1071 (merged), sbroad!466 (merged)