gh-35155: Update the logo at README.md

    
The original logo at the README.md seems broken while accessing from
GitHub's web interface. After careful consideration, I decided not to
use the original black logo (at
/src/doc/common/static/logo_sagemath_black.svg) due to potential
disparities in its visibility when viewed by users with different
display settings, such as light and dark modes. Therefore, I have update
it now to use different variants for dark and light schemes.

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URL: #35155
Reported by: Priyanshu Kumar Rai
Reviewer(s): Kwankyu Lee

README.md

@@ -1,4 +1,9 @@
-<a href="https://sagemath.org"><img src="src/doc/common/static/logo_sagemath_black.svg" height="60" align="right" /></a>
+<a href="https://sagemath.org">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="src/doc/common/static/logo_sagemath_white.svg">
+    <img src="src/doc/common/static/logo_sagemath_black.svg" height="60" align="left">
+  </picture>
+</a>
 
 # Sage: Open Source Mathematical Software