Demo site of the package hosted as Github Pages

[daadu]

Is your feature request related to a problem? Please describe.
Could be used to quickly see what the package can do and interact with it.

Describe the solution you'd like
build example for flutter web and host it as Github pages.

[daadu]

• Possibly should be triggered with Github Actions

[daadu]

Requested for Github Pages setting fluttercommunity/community#55

[daadu]

Demo app hosted at https://fluttercommunity.github.io/backdrop/demo

[WieFel]

@daadu small side note: I get 404 when opening https://github.com/fluttercommunity/backdrop/demo