docs: clarify how to get LangSmith API key (#32402)
**Description:**
I've added a small clarification to the chatbot tutorial. The tutorial
mentions setting the `LANGSMITH_API_KEY`, but doesn't explain how a new
user can get the key from the website. This change adds a brief note to
guide them to the Settings page.
P.S. This is my first pull request, so I'm excited to learn and
contribute!
**Issue:**
N/A
**Dependencies:**
N/A
**Twitter handle:**
@sohamactive
Co-authored-by: Mason Daugherty <mason@langchain.dev>
