feat: add appendix D (observability)

[toddbaert]

• adds small appendix section to spec that defines how particular SDK fields are mapped to the recently merge OTel semantics
• this allows providers and hooks to export OTel data consistently for easy interop
• adds a couple supporting glossary liks

[toddbaert]

I've marked as draft for now since theres some ongoing conversations in OTel about how to represent certain types of event data: open-telemetry/semantic-conventions#1651 (comment)

[lukas-reining]

This looks good to me, we might consider the PR I mentioned regarding the value_type, but I think we could also start with the value as any type and add the type hint from the PR later as it does not change the any type.

[toddbaert]

Merged with just 3 approvals since we have a diverse set and this is just an appendix.