Add new avatar image and update config reference

Replaced the avatar image source in the configuration from 'img/avatar.jpg' to 'img/avatar.png' and added the new PNG avatar image to the assets.

config/_default/params.yml

@@ -18,7 +18,7 @@ sidebar:
   avatar:
     enabled: true
     local: true
-    src: img/avatar.jpg
+    src: img/avatar.png
 
 article:
   math: false