Create a side image for the Tor Browser windows installer

On Tor Browser we still show the default NSIS asset when the installation ends.

It would be nice to show something else (e.g., for MB we have https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/blob/main/projects/browser/windows-installer/mullvadbrowser-release.bmp).

For MB we have channel-specific asset. It would be nice also for Tor Browser, but maybe an initial one for release could be enough as a starter.