DOC: improve README logo in dark mode

[importstring]

This PR improves logo rendering across light and dark themes by switching the displayed asset based on the viewer's color scheme.

Proposed README snippet:

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/pytorch/ignite/master/assets/logo/ignite_logo_mixed_light.svg">
  <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/pytorch/ignite/master/assets/logo/ignite_logo_mixed.svg">
  <img src="https://raw.githubusercontent.com/pytorch/ignite/master/assets/logo/ignite_logo_mixed.svg"
       alt="PyTorch-Ignite logo"
       width="512">
</picture>

Preview of the current and updated rendering:

The first image is the proposed change, and the third is what the logo currently looks like.

[importstring]

This follows the same pattern used by other ML projects. For example, Hugging Face’s README uses a light/dark aware logo where the wordmark color changes with the theme so that it stays readable in both modes. The change here mirrors that behaviour for the PyTorch-Ignite logo.

[vfdev-5]

Thanks for improving that, @importstring !