Add more examples to app.rerun.io, and link to their source code

[emilk]

Right now http://app.rerun.io is minimal, showing one example with no explanation.

I would like to add more examples (eventually all), with HTML buttons to switch between them, and for each example explain what it is doing and link to its source code. The explanatory text should be the same as on https://www.rerun.io/docs/getting-started/examples.

This would be a great way to introduce users to the power of rerun, and also work as a form of get-started guide.

[emilk]

We need to:

• Pick some nice examples
• Build a nice UI

Bonus:

• Make the example selection work on mobile

[emilk]

https://demo.rerun.io/ is up!