Missing README and instructions for contributors

[wna-se]

As a new contributor and first-time user or Jupyter Book the repository is a bit confusing to navigate. We could consider adding a README with links to the relevant documentation, including some minimal instructions to get started (e.g. building the book locally) and how the content is organised. We could also consider creating a folder structure that separates 1) data that is used across several scenarios from content specific to an single scenario; and 2) content that is used in the book from other content, such as examples and documentation.