fix(css): 無駄な横スクロールが出る原因を修正

githubのリボン画像のwidthがはみ出ていた。
元々邪魔なので非表示にした

index.adoc

@@ -12,7 +12,9 @@
 
 ifeval::["{backend}" == "html5"]
 ++++
-<a href="https://github.com/azu/promises-book/" style="position: fixed; top: 0; right: 0;border: 0;"><img style="border: 0;" src="https://camo.githubusercontent.com/38ef81f8aca64bb9a64448d0d70f1308ef5341ab/68747470733a2f2f73332e616d617a6f6e6177732e636f6d2f6769746875622f726962626f6e732f666f726b6d655f72696768745f6461726b626c75655f3132313632312e706e67" alt="Fork me on GitHub" data-canonical-src="https://s3.amazonaws.com/github/ribbons/forkme_right_darkblue_121621.png" /></a>
+<div class="github-ribbon">
+<a href="https://github.com/azu/promises-book/"><img style="border: 0;" src="https://camo.githubusercontent.com/38ef81f8aca64bb9a64448d0d70f1308ef5341ab/68747470733a2f2f73332e616d617a6f6e6177732e636f6d2f6769746875622f726962626f6e732f666f726b6d655f72696768745f6461726b626c75655f3132313632312e706e67" alt="Fork me on GitHub" data-canonical-src="https://s3.amazonaws.com/github/ribbons/forkme_right_darkblue_121621.png" /></a>
+</div>
 ++++
 
 icon:twitter[2x, link=https://twitter.com/search?q=%23Promise%E6%9C%AC] ハッシュタグは https://twitter.com/search?q=%23Promise%E6%9C%AC[#Promise本]

public/css/overload.css

@@ -13,4 +13,17 @@ li > a.sc-current-element:before {
     background-color: #ffa500;
     transition-property: background-color, color;
     transition-duration: 0.3s;
+}
+
+.github-ribbon {
+    position: fixed;
+    z-index: 100;
+    top: 0;
+    right: 0;
+    border: 0;
+}
+@media only screen and (min-width: 768px) {
+    .github-ribbon{
+        display: hidden;
+    }
 }