[MRG + 1] Rebased navbar fix (scikit-learn#7202)

* Fixed the documentation item on the navbars alignment.

* Added z-index to the navbar so it doesn't get covered by page headers

Author: nelson-liu

doc/themes/scikit-learn/static/nature.css_t

@@ -116,6 +116,7 @@ div.navbar div.nav-icon {
         background: #ff9c34;
         padding: 5px 10px;
         border-radius: 5px;
+        z-index: 10;
     }
     div.navbar ul li {
         display: none;
@@ -137,6 +138,7 @@ div.navbar div.nav-icon {
         top: 10px;
         margin-right: 10px;
         background: #ff9c34;
+        z-index: 9;
     }
     div.navbar.responsive > ul li {
         display: flex;
@@ -1186,6 +1188,7 @@ div.navbar ul.dropdown-menu li a.btn {
   vertical-align: middle;
   padding: 0;
   padding-left: 4px;
+  font-size: 8px;
 }
 
 .navbar .btn-group .btn {
@@ -1197,10 +1200,6 @@ div.navbar ul.dropdown-menu li a.btn {
   box-shadow: inset 0 0px 0 rgba(255,255,255,.2), 0 0px 0px rgba(0,0,0,.05);
 }
 
-a.btn.dropdown-toggle,  a.btn.dropdown-toggle:hover{
-  vertical-align: baseline;
-}
-
 li#other-versions {
   position: absolute;
   left: inherit;
@@ -1268,7 +1267,7 @@ li#other-versions {
   color: #fff;
   vertical-align: middle;
   border-top-color: rgb(255, 255, 255);
-  padding-bottom: 8px;
+  padding-bottom: 5px;
 }
 
 .navbar .dropdown-menu .divider {