Add reference application for new Getting Started

[svrnm]

Hey there,

after a few months of silence I finally got back to the project for a new getting started app. I merged the specification for the reference app into the oteldotio repo, and now I am looking for language SIGs that go ahead and implement it:

• opentelemetry.io/docs/getting-started/reference-application-specification
• open-telemetry/opentelemetry.io@main/content/en/docs/getting-started/reference-application-specification.md

Not sure if this belongs here into the core repo or contrib, so please move it accordingly.

I also think this is something that can be picked up by a new contributor.

Thanks!