docs: add LangDB AI Gateway as a tracing provider

[MrunmayS]

Adds LangDB AI Gateway to the External tracing processors list so developers can stream Agent‑SDK traces directly into LangDB’s dashboards.

Highlights

• End‑to‑end observability of every agent step, tool invocation, and guardrail.

• Access to 350+ LLM models through a single OpenAI‑compatible endpoint.

• Quick integration: pip install "pylangdb[openai]"

from pylangdb.openai import init
init()

client = AsyncOpenAI(
    api_key=os.environ["LANGDB_API_KEY"],
    base_url=os.environ["LANGDB_API_BASE_URL"],
    default_headers={"x-project-id": os.environ["LANGDB_PROJECT_ID"]},
)
set_default_openai_client(client)     

• Live demo Thread: https://app.langdb.ai/sharing/threads/53b87631-de7f-431a-a049-48556f899b4d

Fixes #1222

[MrunmayS]

hey @rm-openai! can you take a look at this? TIA