docs - datetimeAdd and datetimeSubtract (#26431)

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

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

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

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

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

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