Overview tutorial (create or polish)

[cwmeijer]

• [x] Rename the demo notebook to Overview
• [x] Edit the Getting started section from the main README. Point to the overview notebook in the main README.
• [x] Split the notebook into 2 sections: explainers and data modalities

Move the Getting Started examples from the main README to the overview notebook

• [x] Text example
• [ ] Image example
• [ ] TS example
• [ ] Tabular example

Below are the original ideas for this issue:

Maybe the demo notebook is a good starting point. Make it a nice generic tutorial. Make sure it is linked to from the right places in our readme and tutorial readme. Consider removing/shortening the Getting Started README section and moving the content to the demo notebook. See also #721.