docs: update links to the manual

In addition to the HTML version, also publish the PDF and Text
versions of our manual.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/docs.html

@@ -2,10 +2,18 @@
 
 <h3>Documentation</h3>
 
-<p>The <a href="/downloads/manual/manual.html">Buildroot user manual</a>
-is where you want to start reading if you wish to understand how
-Buildroot works, or wish to change/extend/fix things. If you find any
-errors (factual, grammatical, whatever) please report them to us.</p>
+<p>The Buildroot user manual is where you want to start reading if you
+wish to understand how Buildroot works, or wish to change/extend/fix
+things. It is available as:</p>
+
+<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>
+
+<p>If you find any errors (factual, grammatical, whatever) please
+report them to us.</p>
 
 <p>If you find that you need help with Buildroot, you can ask for
 help:</p>