Rename _animation.scss to _transitions.scss (#21370)

* Rename _animation.scss to _transitions.scss for accuracy

* fix docs reference to file name

* cleanup line breaks

scss/_transitions.scss

@@ -0,0 +1,34 @@
+.fade {
+  opacity: 0;
+  @include transition($transition-fade);
+
+  &.active {
+    opacity: 1;
+  }
+}
+
+.collapse {
+  display: none;
+  &.active {
+    display: block;
+  }
+}
+
+tr {
+  &.collapse.active {
+    display: table-row;
+  }
+}
+
+tbody {
+  &.collapse.active {
+    display: table-row-group;
+  }
+}
+
+.collapsing {
+  position: relative;
+  height: 0;
+  overflow: hidden;
+  @include transition($transition-collapse);
+}