Add example for providing fallback content for legacy user agents

[marcoscaceres]

It would be great to show how one can provide fallback content for user agents that don't support model. This could include showing a <video> or <picture> instead.

[zachernuk]

There is content in the explainer now (https://github.com/immersive-web/model-element/blob/main/explainer.md#fallback-content) does that mean we just close this? @marcoscaceres @cabanier