Add documentation on datasets and dataloaders

[anwai98]

This PR addresses the following:

• Add type hints for majority of the dataset-related functionalities
• Add a markdown documentation in the torch_em.data.datasets submodule, to briefly explain the available datasets and supported data design choices for creating dataloaders
• (WIP) Add a tutorial notebook on how to create datasets / dataloaders

[anwai98]

@constantinpape This is ready to review from my side!

[constantinpape]

The code changes look good, but we should move the markdown file.

I haven't reviewed the notebook itself yet.

[constantinpape]

I will merge for now. We can update this later when overhauling the datasets.