Document the status of the attribute registry and whether md or yaml are the source of truth

[lmolkova]

Based on the discussion in the https://github.com/open-telemetry/semantic-conventions/pull/1345 it seems we don't have clarity on:

• what's the source of truth yaml or md
• what represents the registry - https://github.com/open-telemetry/semantic-conventions/blob/main/docs/attributes-registry or https://github.com/open-telemetry/semantic-conventions/tree/main/model/registry
• how attribute definition is decoupled from usage (by folder structure or dedicated schema https://github.com/open-telemetry/semantic-conventions/issues/1269)

This becomes much more interesting when we combine it with code-generation and semconv artifact structure.

I believe we need to think through and document answers to these questions