[MLOB-983] Add LLM Observability and Serverless Compatibility Documentation

[sabrenner]

What does this PR do? What is the motivation?

Adds documentation for using LLM Observability in serverless environments. This is following a change in our datadog-lambda-python layer to handle enabling LLM Observability, and providing out-of-the-box force flushing by default, we want users to be able to take advantage of these changes for an easier serverless onboarding experience.

Merge instructions

• [ ] Please merge after reviewing

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/sambrenner/llmobs-serverless/llm_observability/sdk
• https://docs-staging.datadoghq.com/sambrenner/llmobs-serverless/llm_observability/trace_an_llm_application