[editorial] Spec: replace links to semantic_conventions by semconv section of OTel website

[chalin]

Once semantic conventions are published via the OTel website (track via open-telemetry/opentelemetry.io#2721), then links like the following should be replaced by corresponding links into the semantic conventions section of the website:

• https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/resource/semantic_conventions