docs: add wikipedia integration docs (#24932)
Dear langchain maintainers,
I add the wikipedia integration docs according to the [web
docs](https://python.langchain.com/v0.2/docs/integrations/retrievers/wikipedia/),
and follow the format of [tavily
example](https://github.com/langchain-ai/langchain/blob/master/docs/docs/integrations/retrievers/tavily.ipynb)
and [retriever
template](https://github.com/langchain-ai/langchain/blob/master/libs/cli/langchain_cli/integration_template/docs/retrievers.ipynb),
this is my first time contributing large repo. please let me know if I'm
doing anything wrong, thank you!
Topic related: #24908
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Co-authored-by: Chester Curme <chester.curme@gmail.com>
