add everyting for docs

docs/snippets/get_started/quickstart/llm_chain.mdx

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+```python
+from langchain.chat_models import ChatOpenAI
+from langchain.prompts.chat import (
+    ChatPromptTemplate,
+    SystemMessagePromptTemplate,
+    HumanMessagePromptTemplate,
+)
+from langchain.chains import LLMChain
+from langchain.schema import BaseOutputParser
+
+class CommaSeparatedListOutputParser(BaseOutputParser):
+    """Parse the output of an LLM call to a comma-separated list."""
+
+
+    def parse(self, text: str):
+        """Parse the output of an LLM call."""
+        return text.strip().split(", ")
+
+template = """You are a helpful assistant who generates comma separated lists.
+A user will pass in a category, and you should generated 5 objects in that category in a comma separated list.
+ONLY return a comma separated list, and nothing more."""
+system_message_prompt = SystemMessagePromptTemplate.from_template(template)
+human_template = "{text}"
+human_message_prompt = HumanMessagePromptTemplate.from_template(human_template)
+
+chat_prompt = ChatPromptTemplate.from_messages([system_message_prompt, human_message_prompt])
+chain = LLMChain(
+    llm=ChatOpenAI(),
+    prompt=chat_prompt,
+    output_parser=CommaSeparatedListOutputParser()
+)
+chain.run("colors")
+# >> ['red', 'blue', 'green', 'yellow', 'orange']
+```