doc: Link to some useful talks

Talks are a great way to learn about U-Boot and have been lost now that
the Denx Wiki has gone away.

These are stored at elinux.org so link to that .

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/doc/index.rst b/doc/index.rst
index 02de1d4..57b42c6 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -25,6 +25,7 @@
    :maxdepth: 2
 
    build/index
+   learn/index
    usage/index
 
 Developer-oriented documentation