add everyting for docs

docs/snippets/get_started/quickstart/openai_setup.mdx

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+First we'll need to install their Python package:
+
+```bash
+pip install openai
+```
+
+Accessing the API requires an API key, which you can get by creating an account and heading [here](https://platform.openai.com/account/api-keys). Once we have a key we'll want to set it as an environment variable by running:
+
+```bash
+export OPENAI_API_KEY="..."
+```
+
+If you'd prefer not to set an environment variable you can pass the key in directly via the `openai_api_key` named parameter when initiating the OpenAI LLM class:
+
+```python
+from langchain.llms import OpenAI
+
+llm = OpenAI(openai_api_key="...")
+```