Add initial docs

docs/input/assets/bootstrap/_close.scss

@@ -0,0 +1,41 @@
+.close {
+  float: right;
+  @include font-size($close-font-size);
+  font-weight: $close-font-weight;
+  line-height: 1;
+  color: $close-color;
+  text-shadow: $close-text-shadow;
+  opacity: .5;
+
+  // Override <a>'s hover style
+  @include hover() {
+    color: $close-color;
+    text-decoration: none;
+  }
+
+  &:not(:disabled):not(.disabled) {
+    @include hover-focus() {
+      opacity: .75;
+    }
+  }
+}
+
+// Additional properties for button version
+// iOS requires the button element instead of an anchor tag.
+// If you want the anchor version, it requires `href="#"`.
+// See https://developer.mozilla.org/en-US/docs/Web/Events/click#Safari_Mobile
+
+// stylelint-disable-next-line selector-no-qualifying-type
+button.close {
+  padding: 0;
+  background-color: transparent;
+  border: 0;
+  appearance: none;
+}
+
+// Future-proof disabling of clicks on `<a>` elements
+
+// stylelint-disable-next-line selector-no-qualifying-type
+a.close.disabled {
+  pointer-events: none;
+}