Provide guidance on $id keyword

[jack-berg]

More codifying existing practices as we approach stability.

See JSON schema docs on $id and other links in the PR for context.

It may be good to eventually have automation that makes these documents accessible on opentelemetry.io, but I don't think that's strictly necessary. Base on my read of these docs, its sufficient for the URL to be in our control.

It is also best practice to include an $id property as a unique identifier for each schema. For now, just set it to a URL at a domain you control, for example:

Actually requiring upload to opentelemetry.io would require thinking about if we should encode version into the URI scheme and how that affects development in this repo. I.e. is it https://opentelemetry.io/otelconfig/opentelemetry_configuration.json or https://opentelemetry.io/otelconfig/<version>/opentelemetry_configuration.json?

Its not clear to me what we gain by making these URLs resolve on opentelemetry.io. I do think we should open an issue on https://github.com/open-telemetry/opentelemetry.io to reserve the https://opentelemetry.io/otelconfig/ path, in case we need it in the future.

Thoughts?