Do not install dependencies via pip during CI docs build

[nvaytet]

During the Ci docs build, we first create a conda env with pinned package versions (e.g. scipp is pinned to 23.03.2).
The next step in the docs build is to pip install ., which looks at the dependencies in the setup.cfg and installs potentially newer versions from PyPi, thus rendering the pinning in the conda env useless.

By doing pip install --no-deps ., we avoid this and use the packages in the conda env. This will only install the local ess package.