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/_borders.scss

@@ -0,0 +1,30 @@
+//
+// Border-width
+//
+
+// TBD...?
+
+
+//
+// Border-radius
+//
+
+.rounded {
+  @include border-radius($border-radius);
+}
+.rounded-top {
+  @include border-top-radius($border-radius);
+}
+.rounded-right {
+  @include border-right-radius($border-radius);
+}
+.rounded-bottom {
+  @include border-bottom-radius($border-radius);
+}
+.rounded-left {
+  @include border-left-radius($border-radius);
+}
+
+.rounded-circle {
+  border-radius: 50%;
+}