Adding `alt=""` to tell screenreaders to skip these, we don't reliably
have alt content available for onebox images and they tend to be
decorative, so better to omit.
<img width="738" height="151" alt="learn.microsoft.com onebox with the
link whats new in C# 11, there's a purple C# badge to the left of the
link"
src="https://github.com/user-attachments/assets/15bdc830-683f-4c69-a11b-9d54c66fa627"
/>
We already do this in some oneboxes, so this covers some more cases
including the generic.
cb0b8351a9
