Enhance README with responsive image support and add new banner images (#165)

This PR enhances the README with responsive image support and add new
banner images for light and dark modes.


![image](https://github.com/user-attachments/assets/dab92ddd-6765-4d29-99ab-60bc6d128253)

---------

Co-authored-by: Elizabet Oliveira <[email protected]>

Author: elizabetdev

README.md

@@ -1,5 +1,9 @@
 <div align="center">
-  <img src="brand-kit/banner/[email protected]" alt="xata agent logo" />
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="brand-kit/banner/[email protected]">
+    <source media="(prefers-color-scheme: light)" srcset="brand-kit/banner/[email protected]">
+    <img alt="Xata logo" src="brand-kit/banner/[email protected]">
+  </picture>
 </div>
 
 <p align="center">