Use default asciidoctor CSS style instead of doc.css

We tried building Gerrit's documentation using the asciidoctor's
standard CSS style and for many people the result looks better
than Gerrit's default doc.css.

The search box is rendered with fixed position now.  With the
asciidoctor default style sheet the "position:absolute" attribute no
longer worked as intended, because there are other elements in the
page with a "position" attribute. So the search box was placed randomly
somewhere next to a section header and screwed up the text layout
completely.

Test Plan:
$ buck build Documentation:html
$ open buck-out/gen/Documentation/html__tmp/Documentation/index.html

Change-Id: Ia276aeb5c79b9fbd9589db444a83c084f07aee40
Also-by: Michael Ochmann <michael.ochmann@sap.com>
diff --git a/Documentation/replace_macros.py b/Documentation/replace_macros.py
index fec4a58..0930572 100755
--- a/Documentation/replace_macros.py
+++ b/Documentation/replace_macros.py
@@ -60,9 +60,32 @@
 SEARCH_BOX = """
 
 ++++
-<div style="position:absolute; right:20px; top:20px;">
-<input type="text" id="docSearch" size="70" />
-<button type="button" id="searchBox">Search</button>
+<div style="
+  position:fixed;
+  top:0px;
+  right:0px;
+  text-align:
+  right;
+  padding-top:2px;
+  padding-right:0.5em;
+  padding-bottom:2px;">
+<input size="40"
+  style="line-height: 0.75em;font-size: 0.75em;"
+  id="docSearch"
+  type="text">
+<button style="
+  background:none!important;
+  border:none;
+  padding:0!important;
+  vertical-align:bottom;
+  font-family:'Open Sans','DejaVu Sans',sans-serif;
+  font-size:0.8em;
+  color:#1d4b8f;
+  text-decoration:none;"
+  type="button"
+  id="searchBox">
+  Search
+</button>
 <script type="text/javascript">
 var f = function() {
   window.location = '../#/Documentation/' +