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>