🩹 make sure to activate the venv in RtD build

Description

This PR makes sure that the virtual environment in the RtD build is activated so that the installed console-scripts become available.

[x] The pull request only contains commits that are related to it.
[x] I have added appropriate tests and documentation.
[x] I have made sure that all CI jobs on GitHub pass.
[x] The pull request introduces no new warnings and follows the project's style guidelines.