also remove the overflow-x. when you specify either overflow-y or overflow-x, the other is essentially nullified and thus ineffective anyway

fixes #22977

The Japanese website (http://bootstrap4.jp/) doesn't exist therefore I removed the link to it.

- Moves the embed SCSS to the utilities subfolder
- Separate the floats from the embed docs
- Rename embed docs from responsive helpers to embeds

Use `display: block` since we're not really using any flex styles on the `.carousel-items` (only their controls and indicators). For the indicators, since moving to `width`, our indicators grow to fill available space, so we need to change `flex: 1 0 auto` to `flex: 0 1 auto` to prevent that.