Move notebooks into separate repo

[Marius1311]

Description of feature

Notebooks are currently stored in site/vignettes, which inflates the repo a bit. The size of that vignettes folder is 31MB, whereas the size of the entire code in src is only 104KB. This makes installing the package slower, and is also slows down GitHub action runs. Ideally, we would move all notebooks into a separate repository, which is then included in this repo as a sub-module, see e.g. the setup in moscot: https://github.com/theislab/moscot_notebooks

[Marius1311]

That's also reflected in the language overview on GitHub, the actual code makes up about 0.2% of this repo.