Add more details about "service" to the Resource page

[svrnm]

With service.name having a clear description now (open-telemetry/semantic-conventions#630) and service.type coming (open-telemetry/semantic-conventions#575), we should expand on this important topic on our concept pages (https://opentelemetry.io/docs/concepts/resources/#semantic-attributes-with-sdk-provided-default-value)

[cartermp]

I'm thinking we're also due for another rewording of this to clarify what resources are exactly. One thing I've noted seems to work well with people is framing it up explicitly like this:

Spans, Logs, Metrics, and their attributes tell you what is happening.
Resources tell you where it is happening.

There's...a little bit of nuance lost there, like how a span also kinda tells you the "where", and how a resource isn't just about concrete things, but can represent a virtual entity like a service. But it's a framing that I've seen work well.