doc: Add a description for bootmeth_extlinux

Add documentation for the extlinux bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/doc/develop/bootstd/index.rst b/doc/develop/bootstd/index.rst
index f4f87c7..5bbb3d6 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    overview
+   extlinux