add copy codeblock button

Apply suggestions from code review

Co-authored-by: Danny <Rapptz@users.noreply.github.com>

Change to icon, change according to slice's review

docs/_static/style.css

@@ -143,7 +143,7 @@ label.toggle input:checked + span.toggle-slider:before {
   transform: translateX(18px);
 }
 
- 
+
 div.related {
   padding: 10px 10px;
   width: 100%;
@@ -703,4 +703,30 @@ div.code-block-caption {
     background-color: transparent;
     border-left: none;
   }
-}
+}
+
+.relative-copy {
+  position: relative;
+}
+
+.copy {
+  cursor: pointer;
+  position: absolute;
+  width: 16px;
+  height: 16px;
+  top: 0px;
+  right: 0px;
+  border: 1px solid #C6C9CB;
+  line-height: 0.8em;
+  font-size: 0.9em;
+  font-family: monospace;
+  padding-left: 0.2em;
+  padding-right: 0.2em;
+  border-radius: 0px 3px 0px 0px;
+  text-align: center;
+}
+
+.copy i {
+  display: inline;
+  vertical-align: middle;
+}