Add new border utils

- Move and rename .img-rounded to .rounded, .img-circle to .rounded-circle
- Add new .rounded-{direction} utils
- New docs pages for border utils with TBD comments for the border property
- Removes most image examples for rounding from the content/images docs in favor of new docs page

scss/_utilities.scss

@@ -1,4 +1,5 @@
 @import "utilities/background";
+@import "utilities/borders";
 @import "utilities/clearfix";
 @import "utilities/display";
 @import "utilities/flex";