Move `gap` utility API from "Flex" to "Spacing"
Move `gap` utility API from "Flex" to "Spacing"

The `gap` utility is described on the [Spacing page][1] but is not
actually mentioned on the [Flex page][2] (apart from the [API
section][3]).

[1]: https://getbootstrap.com/docs/5.1/utilities/spacing/
[2]: https://getbootstrap.com/docs/5.1/utilities/flex/
[3]: https://getbootstrap.com/docs/5.1/utilities/flex/#utilities-api

inway/bootstrap,twbs/bootstrap,coliff/bootstrap,inway/bootstrap,coliff/bootstrap,tjkohli/bootstrap,tjkohli/bootstrap,m5o/bootstrap,twbs/bootstrap,inway/bootstrap,m5o/bootstrap