Add Datadog Integration Documentation

[sabrenner]

Adds Datadog LLM Observability integration documentation:

1. Describes what LLM Observability is
2. Outlines setup steps (account, API key, environment variables for instrumentation)
3. Provides an example for getting a quick LLM Observability trace in the Datadog LLM Observability UI, describing some additional features
4. Provides some references to official Datadog documentation and an set of Datadog CrewAI instrumentation docs.

[barieom]

lgtm!

[greysonlalonde]

@tonykipkemboi gtg on this?

[tonykipkemboi]

@barieom can you add translated versions of the .mdx docs for ko and pt-BR under their specific folders then do the same on the docs.json so the docs support all the three languages?