readme: update method to show theme-based logo

see https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

README.md

@@ -1,9 +1,10 @@
 <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 href="https://nixos.org">
+    <picture>
+      <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png">
+      <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos-white.png">
+      <img src="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png" width="500px" alt="NixOS logo">
+    </picture>
   </a>
 </p>