Add search-bar Go button.

doc/.sphinx/_static/custom.css

@@ -193,3 +193,19 @@ details summary {
 .highlight button.copybtn {
    opacity: 0;
 }
+
+/* Mimicking the 'Give feedback' button for UX consistency */
+.sidebar-search-container input[type=submit] {
+    color: #FFFFFF;
+    border: 2px solid #D6410D;
+    padding: var(--sidebar-search-input-spacing-vertical) var(--sidebar-search-input-spacing-horizontal);
+    background: #D6410D;
+    font-weight: bold;
+    font-size: var(--font-size--small);
+    cursor: pointer;
+}
+
+.sidebar-search-container input[type=submit]:hover {
+    text-decoration: underline;
+}
+

doc/.sphinx/_templates/sidebar/search.html

@@ -0,0 +1,7 @@
+<form class="sidebar-search-container" method="get" action="{{ pathto('search') }}" role="search">
+    <input class="sidebar-search" placeholder="{{ _("Search") }}" name="q" aria-label="{{ _("Search" ) }}">
+    <input type="submit" value="Go">
+    <input type="hidden" name="check_keywords" value="yes">
+    <input type="hidden" name="area" value="default">
+  </form>
+  <div id="searchbox"></div>