Iterate on border utilities (#36239)

* Remove `--bs-border-opacity: 1` from `.border-*` utilities

We set `--bs-border-opacity: 1` globally at the `:root` level, so redeclaring it on every `.border-*` utility doesn't make much sense. I think we can drop this.

* Remove global border-opacity var, restore on .border-color classes, move .border-color utils down the list to fix some specificity issues

* Add some demos of border utils to the docs