[chore] add link to Datadog Extension documentation (#43800)

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#### Description
adds link to README.md for Datadog Extension

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#### Documentation
README only
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Author: jackgopack4

extension/datadogextension/README.md

@@ -63,5 +63,5 @@ extensions:
 - `tls`: TLS settings for outbound connections.
 
 ## Notes
-- The extension is in active development. Functionality and configuration options may change as Datadog infrastructure monitoring features evolve.
-- For the latest updates and UI changes, watch the [Datadog Monitor blog](https://www.datadoghq.com/blog/) and the [OpenTelemetry Collector Contrib repository](https://github.com/open-telemetry/opentelemetry-collector-contrib).
+- The extension is in active development. Functionality and configuration options may change as Datadog OpenTelemetry monitoring features evolve.
+- Please see [official documentation on Datadog's website](https://docs.datadoghq.com/opentelemetry/integrations/datadog_extension/) for more details.