feature: add option for docs-next in guide (#2644)

* add option for next in guide

* docs: update version label

Co-authored-by: Ben Hong <ben@bencodezen.io>

Author: acidiney

themes/vue/layout/partials/sidebar.ejs

@@ -21,6 +21,7 @@
           <%- titles[type] %>
           <% if (['cookbook', 'style-guide'].indexOf(type) === -1) { %>
             <select class="version-select">
+              <option value="v3">3.x-beta</option>
               <option value="SELF" selected>2.x</option>
               <option value="v1">1.0</option>
               <option value="012">0.12</option>