docs: adjust the 'Use Cases' section in the overview

- Add IPSec use case into 'Supported Use Cases' section
- Remove IPSec use case from 'Future Use Cases' section

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

Change-Id: I70c2130f969158faf83deee9225ac7d755a21f42