docs - convertTimezone, datetimeAdd, datetimeSubtract, and datetimeDiff (#26685)

docs/questions/query-builder/expressions-list.md

docs/questions/query-builder/expressions/converttimezone.md

docs/questions/query-builder/expressions/datetimeadd.md

docs/questions/query-builder/expressions/datetimediff.md

docs/questions/query-builder/expressions/datetimesubtract.md

frontend/src/metabase-lib/expressions/helper-text-strings.ts