Change docs link from stable/ to latest/ (googleapis#3763)

Author: tswast

index.html

@@ -92,7 +92,7 @@ <h2>One-line install</h2>
     <div class="container">
       <ul class="featuring-links">
         <li>
-          <a href="stable/" title="google-cloud-python docs" class="btn btn-docs">
+          <a href="latest/" title="google-cloud-python docs" class="btn btn-docs">
             <img src="_landing-page-static/images/icon-lang-python.svg" alt="Python icon" />
             Read the Docs
           </a>