Use strands logo that looks good in dark & light mode (#44)

Similar to strands-agents/sdk-python/pull/475 but using a dedicated github icon.

The github icon is the lite logo but copied/renamed to make it dedicated to github

Author: zastrowm

README.md

@@ -1,7 +1,7 @@
 <div align="center">
   <div>
     <a href="https://strandsagents.com">
-      <img src="https://strandsagents.com/latest/assets/logo-auto.svg" alt="Strands Agents" width="55px" height="105px">
+      <img src="https://strandsagents.com/latest/assets/logo-github.svg" alt="Strands Agents" width="55px" height="105px">
     </a>
   </div>