docs: import Sphinx Tabs extension

- Import Sphinx Tabs extension.
- Use Sphinx tabs to describe wrk2 building on non-DUT node.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: I3de6f40dcdc52c4b4c71d2d94b6fcdb488a80b52