readme: update online docs link

Only the release version of project online document [2] is accessible to
public, and the latest version link [1] is made invisible.

The online document link [0] will be redirected to the lastest release
version of document, which is the v22.06 release [2] as of now.

[0] https://dataplane-stack.docs.arm.com
[1] https://dataplane-stack.docs.arm.com/en/latest
[2] https://dataplane-stack.docs.arm.com/en/nw-ds-2022.06.30/



Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Yefeng Lin <Yefeng.Lin@arm.com>
Reviewed-by: Liangxing Wang <Liangxing.Wang@arm.com>
Change-Id: I4e40e83c9b03d82773706c03560b66c3d5bc80ea