docs: Improve introduction to Edit Prediction (#26620)

As I was writing a blog post about Edit Prediction, I realized we didn't
have a great section in the docs I could link to talking about
configuring it. We weren't: 1) explicitly exposing the settings code to
add Zed as the edit prediction provider, and 2) not showing an image of
the title bar banner.

Release Notes:

- N/A