docs: use dark-mode logo in README (#3821)

Adds a light logo to header to appear in dark mode so it doesn't appear almost empty.

Author: adamlui

README.md

@@ -1,4 +1,10 @@
-# [<img src="https://katex.org/img/katex-logo-black.svg" width="130" alt="KaTeX">](https://katex.org/)
+<h1><a href="https://katex.org/">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="https://katex.org/img/katex-logo.svg">
+    <img alt="KaTeX" width=130 src="https://katex.org/img/katex-logo-black.svg">
+  </picture>
+</a></h1>
+
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 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/semantic-release/semantic-release)
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