doc: use absolute images for image links in README

Created by: bobheadxi

This allows our README to render correctly in more environments, most notably Sourcegraph itself.

Sourcegraph doesn't have a homegrown mechanism for serving images 😞 the /-/raw endpoint specifically does not allow images to be served and rendered. Details: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@main/-/blob/cmd/frontend/internal/app/ui/raw.go?L200%3A3-205%3A80= That means that we won't be able to get the Sourcegraph logo to render without updating the image source in the README to use a full URL rather than a relative URL

Right now, images just fail to load:

Test plan

n/a