[MLOB-1326] Add Node.js LLM Observability SDK docs

[sabrenner]

What does this PR do? What is the motivation?

Adds Node.js SDK documentation for LLM Observability

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/sam.brenner/llmobs-node-sdk/llm_observability/setup/sdk/nodejs
• https://docs-staging.datadoghq.com/sam.brenner/llmobs-node-sdk/llm_observability/setup/sdk/python

Modified Files

• https://docs-staging.datadoghq.com/sam.brenner/llmobs-node-sdk/llm_observability/setup/auto_instrumentation
• https://docs-staging.datadoghq.com/sam.brenner/llmobs-node-sdk/llm_observability/setup/sdk/

[neko-dd]

Created DOCS-9395 for docs team review

[sabrenner]

@cswatt Thanks! I'll ping you directly, we're still waiting on the actual release to happen from dd-trace-js, once it's release we're good to go to merge. Should be this afternoon, but I'll keep you posted