fix(v1): Fix page title render issue when referred by search result

When Algolia DocSearch query finds a match for a page's title, it attempts to generate
a permalink. Because the page title element (`h1`) does not have an `id`, Algolia
generates uses the `id` from closes parent element. Because of this, the page title
scrolls to a position that is slightly overlayed by the fixed top navigation bar.

This fix sets an `id` for the page title so that the search result is able to generate
a more accurate permalink.

packages/docusaurus-1.x/lib/core/Doc.js

@@ -238,7 +238,9 @@ class Doc extends React.Component {
         <header className="postHeader">
           {editLink}
           {!this.props.hideTitle && (
-            <h1 className="postHeaderTitle">{this.props.title}</h1>
+            <h1 id="__docusaurus" className="postHeaderTitle">
+              {this.props.title}
+            </h1>
           )}
         </header>
         <article>