Add some placeholder images/pages

[tacman]

Description

It'd be nice to have some placeholder images that can be overridden and customized but worked out of the box

For example.

            offline_fallback:
               
                image: 'images/offline.svg'

We'd need to find some open source or cc-no-attribution files, of course.

Similarly, we could add some default pages for offline at bundle level. I'll experiment with that in pwa-extra-bundle.

Example

No response

[Spomky]

The problem is that if any of the offline_fallback option is set, then the offline feature is enabled. Some users may want to keep it disabled.

[tacman]

How about adding the assets to the repo, but commenting them out in the yaml.

In particular, an offline.svg in the bundle would safe the effort of looking one up when the default might be fine.