Favicons with white bg for both light and dark mode

[chalin]

• Fixes Favicon difficult to see in Chrome when in dark mode  #117
• Adds a singe .ico file with white background using https://cthedot.de/icongen as recommended by the Docsy UG:

  

  Not as good as a manually created icon, but I think that this is good enough, and works in both light and dark modes.

WDYT @lukpueh et all?