docs.html: add links to nightly build of manual as well

The html/pdf/text version of the manual is regenerated from current
git head every 6 hours.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/docs.html

@@ -6,12 +6,20 @@
 wish to understand how Buildroot works, or wish to change/extend/fix
 things. It is available as:</p>
 
+<h4>Latest stable release</h4>
 <ul>
   <li><a href="/downloads/manual/manual.html">Online HTML</a></li>
   <li><a href="/downloads/manual/manual.pdf">PDF document</a></li>
   <li><a href="/downloads/manual/manual.text">ASCII text</a></li>
 </ul>
 
+<h4>Nightly build</h4>
+<ul>
+  <li><a href="http://nightly.buildroot.org/manual.html">Online HTML</a></li>
+  <li><a href="http://nightly.buildroot.org/manual.pdf">PDF document</a></li>
+  <li><a href="http://nightly.buildroot.org/manual.text">ASCII text</a></li>
+</ul>
+
 <p>If you find any errors (factual, grammatical, whatever) please
 report them to us.</p>