docs: add docs on StrOutputParser (#28089)
Think it's worth adding a quick guide and including in the table in the
concepts page. `StrOutputParser` can make it easier to deal with the
union type for message content. For example, ChatAnthropic with bound
tools will generate string content if there are no tool calls and
`list[dict]` content otherwise.
I'm also considering removing the output parser section from the
["quickstart"
tutorial](https://python.langchain.com/docs/tutorials/llm_chain/); we
can link to this guide instead.
