Fix documention header styles (react-navigation#1742)

Author: Paul McBride

website/src/App.css

@@ -27,6 +27,10 @@ body, #root, html {
 .md-header:hover a {
   visibility: visible;
 }
+.link-header a:first-of-type {
+  visibility: visible;
+  color: #182026;
+}
 .nextLink {
   float: right;
 }

website/src/MDPage.js

@@ -64,8 +64,10 @@ const MDPage = ({ navigation, docPath }) => (
           })
           .join('-');
         const Header = getHeadingForLevel(level);
+        const linkHeader = id ? '' : 'link-header';
+        const className = `md-header ${linkHeader}`;
         return (
-          <Header id={id} className="md-header">
+          <Header id={id} className={className}>
             {children} <a href={`#${id}`} title={children}>#</a>
           </Header>
         );