Prep for v5.2.0 docs rename by adding v5.1.3 to version picker

site/content/docs/5.1/components/placeholders.md

@@ -4,6 +4,7 @@ title: Placeholders
 description: Use loading placeholders for your components or pages to indicate something may still be loading.
 group: components
 toc: true
+added: "5.1"
 ---
 
 ## About

site/content/docs/5.1/layout/css-grid.md

@@ -4,7 +4,7 @@ title: CSS Grid
 description: Learn how to enable, use, and customize our alternate layout system built on CSS Grid with examples and code snippets.
 group: layout
 toc: true
-added: 5.1
+added: "5.1"
 ---
 
 Bootstrap's default grid system represents the culmination of over a decade of CSS layout techniques, tried and tested by millions of people. But, it was also created without many of the modern CSS features and techniques we're seeing in browsers like the new CSS Grid.

site/layouts/partials/docs-versions.html

@@ -9,6 +9,9 @@
   <ul class="dropdown-menu dropdown-menu-end" aria-labelledby="bd-versions">
     <li><h6 class="dropdown-header">v5 releases</h6></li>
     <li><a class="dropdown-item current" aria-current="true" href="/docs/{{ .Site.Params.docs_version }}/">Latest ({{ .Site.Params.docs_version }}.x)</a></li>
+    <li>
+      <a class="dropdown-item" href="https://getbootstrap.com/docs/5.1/{{ $group_slug }}/{{ $page_slug }}/">v5.1.3</a>
+    </li>
     <li>
       {{- if eq .Page.Params.added "5.1" }}
         <div class="dropdown-item disabled">v5.0.2</div>