DOC: Replace pandas logo with a conditional image for light and dark … (#57223)

DOC: Replace pandas logo with a conditional image for light and dark themes

Replaced the static pandas logo with a `picture` tag that displays a different logo based on the color scheme. In light mode, the original logo is shown, while in dark mode, a white version of the logo is displayed for better visibility.

Author: LakshmanKishore

README.md

@@ -1,6 +1,7 @@
-<div align="center">
-  <img src="https://pandas.pydata.org/static/img/pandas.svg"><br>
-</div>
+<picture align="center">
+  <source media="(prefers-color-scheme: dark)" srcset="https://pandas.pydata.org/static/img/pandas_white.svg">
+  <img alt="Pandas Logo" src="https://pandas.pydata.org/static/img/pandas.svg">
+</picture>
 
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