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/_images.scss

@@ -9,10 +9,6 @@
   @include img-fluid();
 }
 
-// Rounded corners
-.rounded {
-  @include border-radius($border-radius-lg);
-}
 
 // Image thumbnails
 .img-thumbnail {
@@ -27,11 +23,6 @@
   @include img-fluid(inline-block);
 }
 
-// Perfect circle
-.img-circle {
-  border-radius: 50%;
-}
-
 //
 // Figures
 //