doc: update paths in various user guide

SSL Proxy and TCP Termination user guides had references to
"<nw_ds_workspace>/usecase", which should be
"<nw_ds_workspace>/dataplane-stack/usecase".

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Yefeng Lin <yefeng.lin@arm.com>
Change-Id: I2ded2467160c044c59f4c4919a4fb8d3b0d26068