Add in Datadog LLM Observability Documentation

[sabrenner]

This PR adds in Datadog LLM Observability documentation for instrumenting stacks using the Google ADK. Datadog has released auto-instrumentation support for this in the ddtrace python package within the past couple of months, and are looking to add our presence here as part of the suite of existing listed observability vendors.

Please let me know what extra work is needs to be done on this as applicable and I am happy to iterate and push up changes! Thanks to reviewers in advance!