Rename logo file (#801)

Apologies it seems that I forgot to rename the logo on #800 

With this change the logo shows up on [the
branch](https://github.com/insightsengineering/chevron/tree/renaming_logo%40main):


![image](https://github.com/user-attachments/assets/e034f742-b852-499c-9e62-3efefb072421)