docs: switch to static PNG with syntax highlighting

Replace animated GIF with a static before/after image as
requested by maintainer. 75KB PNG vs 1MB GIF.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: oferlmntr

README.md

@@ -6,7 +6,7 @@
 
 Encode failure into your program.
 
-![Demo](assets/demo.gif)
+![Demo](assets/demo.png)
 
 This package contains a `Result` type that represents either success (`Ok`) or failure (`Err`).