docs(traceai): add Overview and Quickstart

[abhijaisrivastava15]

What

Starts the traceAI section Khushal asked for: an Overview and a Quickstart, the two pages he wants to align on before we build out the rest.

Pages added

• src/pages/docs/traceai/index.mdx — Overview: what traceAI is, what it captures, the four languages + 50+ frameworks, and how it fits with Observe / any OTel backend
• src/pages/docs/traceai/quickstart.mdx — Quickstart: install an instrumentor, register a project, trace one OpenAI call, confirm it in Observe (Python + JS/TS)

Grounding

Every claim is from source, not another doc:

• Definition, languages (Python 3.10+, TS 5.0+, Java 17+, C# .NET), 50+ frameworks, OTel-native, backend-agnostic — future-agi/traceAI README
• Install / register(project_type=ProjectType.OBSERVE, ...) / FI_API_KEY + FI_SECRET_KEY / Transport.GRPC — traceAI README + the shipped Observe quickstart (same verified pattern)

Notes for you (Khushal)

• Sidebar is left to you — I did not touch navigation.ts. Pages are live by URL for review: /docs/traceai and /docs/traceai/quickstart. Titles will sync to whatever sidebar labels you pick.
• Paired in one PR on purpose — the two cross-link, so shipping the Overview alone would 404 the Quickstart card. Happy to split if you prefer.
• IA question: the Quickstart overlaps the existing /docs/observe/quickstart by design (traceAI SDK-first vs Observe platform-first). Want me to keep both, or make Observe's point here?
• Java + C# are named as supported; I only wrote the Python + JS/TS code paths I could verify. Their quickstart snippets come next once you greenlight the section.

Preview

npm run dev -- --port 4444

Then: http://localhost:4444/docs/traceai and http://localhost:4444/docs/traceai/quickstart