[DOCS] add search box (#20251)

This adds a search box (powered by DuckDuckGo site search) to the
navbar.

_layouts/default.html

@@ -35,6 +35,16 @@ common-js:
               <li><a href="{{target |strip }}">{{ link.first }}</a></li>
             {% endfor %}
           </ul>
+          <form class="navbar-form pull-right" role="search" method="get" action="https://duckduckgo.com/">
+            <input type="hidden" name="sites" value="geth.ethereum.org">
+            <input type="hidden" name="kz" value="-1">
+            <div class="input-group navbar-input-group-fixup">
+              <input class="input-sm" type="text" name="q" placeholder="Search site...">
+              <span class="input-group-btn">
+                <button type="submit" class="btn btn-default btn-sm"><i class="fa fa-search" aria-hidden="true"></i></button>
+              </span>
+            </div>
+          </form>
         </div>
       </div>
     </nav>

static/styles/custom/common.css

@@ -50,3 +50,7 @@ table thead tr th, table tbody tr td {
     border-bottom: 1px solid #ddd;
     padding: 4px;
 }
+
+.navbar-input-group-fixup {
+    margin-top: 5px;
+}