Update to new Au logo (#277)

This brings us in line with Aurora's branding refresh.  These new assets
were designed directly by Ethan Bennett.

I tested by running `au-docs-serve` and checking that the favicon was
updated, and by pushing the branch remotely and viewing the rendered
README.  No other assets needed to be updated (`aurora-logo.png`, used
in our AVdocs, is white-on-transparent and was thus unaffected by the
change.)

Before:


![image](https://github.com/user-attachments/assets/77724a34-9956-428c-b8b2-40c0b5879b95)

After:


![image](https://github.com/user-attachments/assets/eadb05e8-570b-448a-ab44-a3d9a672477f)