Investigate solution for glTF-Assets-Viewer when referrer-policy is more strict

The way assets are being view using the view link in the readmes in the output folder will no longer work in future versions of browsers where security will be tighter. We need to investigate a different way to do this.

https://github.com/bghgary/glTF-Assets-Viewer/blob/master/index.js#L241

`document.referrer` will no longer return the whole address of the referrer.

See https://codereview.chromium.org/684683003/ for some context.

Start with glTF-Assets-Viewer/index.js at line 241 and inspect how document.referrer is used by the view links in generated README files. Review the linked Chromium context and determine an alternative that remains compatible with stricter referrer policies. Done means the approach is documented and the affected viewer/readme flow is updated or clearly specified.

Written by the indexing model from the issue text.