doc: cmd: wget: add documentation

Add documentation for the wget command.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index d6283fa..0bc8288 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -79,6 +79,7 @@
    cmd/ums
    cmd/ut
    cmd/wdt
+   cmd/wget
    cmd/xxd
 
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