onlinedocs: index.html: Make 2.3.0 "release preview" visible by the public.

Author: sprintersb

docs/index.html

@@ -50,6 +50,10 @@ <h2>Documentation</h2>
     <td><a href="avr-libc-user-manual/index.html">HTML online</a></td>
   </tr>
   <tr><td colspan=4><hr/></td></tr>
+  <tr>
+    <td>v2.3.0git</td>
+    <td colspan="3"><a href="avr-libc-user-manual-2.3.0/index.html">HTML online</a> (release preview)</td>
+  </tr>
   <tr>
     <td>v2.2.0</td>
     <td><a href="avr-libc-user-manual-2.2.0/index.html">HTML online</a></td>