clarify some docs, improve layout

docs/_includes/nav-docs.html

@@ -1,6 +1,6 @@
 <header class="navbar navbar-static-top bd-sidebar" id="top" role="banner">
   <a href="{{ site.baseurl }}/" class="navbar-brand">
-    Bootstrap
+    Bootstrap v4 alpha
   </a>
 
   <button class="navbar-toggler hidden-sm-up" type="button" data-toggle="collapse" data-target="#docsNavbarContent">

docs/_includes/nav-home.html

@@ -1,5 +1,5 @@
 <header class="bd-header" role="banner">
-  <a class="bd-header-mark" href="{{ site.baseurl }}">Bootstrap</a>
+  <a class="bd-header-mark" href="{{ site.baseurl }}">Bootstrap 4 alpha</a>
 
   <nav class="nav nav-pills bd-header-nav">
     <a class="nav-link" href="{{ site.baseurl }}/getting-started/" onclick="ga('send', 'event', 'Navbar', 'Community links', 'Docs');">Documentation</a>