Add a "social media preview" to GitHub repo

GitHub repos have a setting for "social media preview", which TFQ currently does not make use of:

<img width="642" alt="image" src="https://github.com/user-attachments/assets/29fd7067-18fa-4313-990e-e39696d5382d">

We should add one, to enhance TFQ's visibility. This won't be hard because we already have an icon and logo, so it's just a matter of sizing & cropping something.