Add live demo to index

[pavolloffay]

I think it's worth to add a button Live Demo next to Get Started so people can quickly try Jaeger.

Because the live demo requires more information - the button could point to a new page "Live demo" with brief instructions and a link to hotrod blog post. Or we could embed the blog post directly on the page. The page could also contain information on how to configure tracer to send data to this instance.

Related issue: jaegertracing/jaeger#743