Move documentation from gh-pages branch to 'docs' folder

To prepare the migration to 3.0, we need to change the location where the documentation is generated from.
Having a gh-pages branch makes it hard to synchronize the code and the docs. Having a "docs" folder in the repo itself simplifies this.

docs/_includes/banner.html

@@ -0,0 +1,9 @@
+<div class="site-banner">
+    <div class="wrapper vertical-align-middle">
+        <p class="tag-line">
+            Read and write spreadsheets
+            <br />
+            <strong>quickly</strong> and <strong>at scale</strong>
+        </p>
+    </div>
+</div>