Add branding to login screen

[Vlatombe]

This demonstrates branding of the login/register screen made possible via jenkinsci/jenkins#8462

(I'm not by far a UI designer)

Testing done

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[jonesbusy]

What about to merge this example on the design library ?

I've got a PR to implement custom branding on the login-theme plugin : jenkinsci/login-theme-plugin#70

As far as I know there is no other public consumer of the new APIs

[Vlatombe]

@jonesbusy Good point, I had forgotten about this PR, updated, and now ready to be reviewed/merged.

[timja]

I don't think this belongs here, as people could install this into actual instances, there's no reason I can see to for this to exist outside an initial draft PR.

This belongs in the login-theme plugin imo