docs: Fix sphinx-tabs version in docs-builder

There is an incompatibility issue between the sphinx version in Ubuntu
focal and the docutils version pulled in by sphinx-tabs. We need to make
sure that pip doesn't pull in a version of docutils that would break
sphinx:

https://github.com/sphinx-doc/sphinx/issues/9841



Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>