Bootstrapのpとかmとか

これ何だっけ…？と毎回忘れてしまうのでメモ。
padding とか margin とかのspecing用のクラスです。

What does the `p-N` and its variants represent in Bootstrap v4?
https://stackoverflow.com/questions/42160532/what-does-the-p-n-and-its-variants-represent-in-bootstrap-v4

Spacing
https://getbootstrap.com/docs/4.1/utilities/spacing/