Improve user guidance on how to license Bevy logo

[BD103]

As far as I can tell, the Bevy logo does not have an explicit license. Please see bevyengine/bevy_github_ci_template#45 for the reason it was removed from bevy_ci_template.

In this scenario, @cart will either have to sign-off that all people who use this template may use the logo in the splash screen, or it should unfortunately be removed.

[janhohenheim]

@alice-i-cecile I think we talked about its usage being fine for a splash screen as long as it's properly credited, but @BD103 is right, it's not CC0.
I think I remember it does since belong to the Bevy Foundation.
Which license should we use?
"All rights reserved to the Bevy Foundation, used with permission"? Does the permission extend to users of the template who use it as their splash?