Make the general app outline nicer

[ekatef]

• [x] Rename the index page to a more sensible name keeping in mind the page purpose
• [ ] Make the side bar nicer (see which options does streamlit provide)
• [ ] Add repo name to top left/ panel / sidebar
• [ ] Add logo e.g. Open Energy Transition
• [ ] Add GitHub link (with little github emoji), see PyPSA-Earth documentation
• [ ] Add some nice panel / link to Readme.md (linking to the documentation etc)
• [x] Other details that we can trial: color of panel? Emoji's? question-mark notes?