Add banner image to PPT repo

[delucchi-cmu]

May be visible via "social preview".

We need an image, tho...

[olivialynn]

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/customizing-your-repositorys-social-media-preview

We'll need to track down the snake image

[delucchi-cmu]

[drewoldag]

Does the snake have a name? "sLINCCy", perhaps?