fix: use relative path for README og-image

Move og-image.png to .github/ and use relative path in README for
reliable rendering on GitHub.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ryo-manba

.github/og-image.png

@@ -1,6 +1,6 @@
 <div align="center">
 
-<a href="https://ryo-manba.github.io/data-chart/"><img src="https://raw.githubusercontent.com/ryo-manba/data-chart/main/docs/public/og-image.png" alt="data-chart" width="100%" /></a>
+<a href="https://ryo-manba.github.io/data-chart/"><img src=".github/og-image.png" alt="data-chart" width="100%" /></a>
 
   <h1>data-chart</h1>