Adjust nox config for building the translated site versions

[webknjaz]

I was playing with building the localized sites on RTD and noticed that the *.po files are not being picked up. So I dug deeper and now realized that Sphinx looks for the locales/ dir relative to conf.py and fixed that.

Additionally, since there was no way to invoke nox locally for building the docs in other languages, I've adjusted the nox file to bypass any posargs to the underlying invocation of sphinx-build and now it's possible to do this. For example:

$ python -m nox -s build -- -D language=uk
nox > Running session build
nox > Creating virtual environment (virtualenv) using python3 in .nox/build
nox > python -m pip install -r requirements.txt
nox > sphinx-build --color --keep-going -j auto -b html -n -W -D language=uk source build
Running Sphinx v4.0.3
loading translations [uk]... done
...