Support dark-mode GitHub users

[datosh]

Is your feature request related to a problem? Please describe.
The logo in README.md has a very low contrast ratio (1.37:1) for dark mode users.

Describe the solution you'd like
We could add a dark mode friendly version of the image to the repository and conditionally load the correct one, using:

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="docs/images/logo-dark-mode.png">
  <source media="(prefers-color-scheme: light)" srcset="docs/images/logo.png">
  <img alt="Badrobot Logo" src="docs/images/logo.png">
</picture>

Describe alternatives you've considered
Update logo to look good in both modes.

Additional context

[06kellyjac]

In GitHub you use #gh-light-mode-only #gh-dark-mode-only

<p align="center">
  <a href="https://nixos.org#gh-light-mode-only">
    <img src="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png" width="500px" alt="NixOS logo"/>
  </a>
  <a href="https://nixos.org#gh-dark-mode-only">
    <img src="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos-white.png" width="500px" alt="NixOS logo"/>
  </a>
</p>

https://github.com/NixOS/nixpkgs/blob/master/README.md?plain=1#L1-L8