Stop installing unncessary dependencies in CI

For example, our CI logs show Installing pip packages: sphinx-book-theme>=1.1.0, sphinx-tippy, .. in nox jobs, which seems like a waste of time.

There are multiple places for improvement:

• The doc dependencies are installed for all jobs, while only required for the run docs job.
• The nox jobs install the base environment first, which is then overwritten by the nox specific versions.