Fix logo for documentation

[ahelm]

Following assets are required for the documenation:

• [ ] Logo
  • [ ] Check what dimension are needed
  • [ ] Check if we should use svg or png
• [ ] Favicon
  • [ ] Makes sure the dimension are properly set (requires 24x24?!?)
• [ ] Additional icons
• [ ] Maybe we should and can use a custom nata icon in our documentation?

Check the documentation to add additional logos and icons.