input: Write a function to search for documents containing a specific keyword.
  assert search(["Hello world!", "Python is great.", "Hello Python world!"], "hello") == ["Hello world!", "Hello Python world!"]
  assert search(["Data science is fun.", "Machine learning is the future.", "AI is fascinating."], "learning") == ["Machine learning is the future."]
  assert search(["Cats are cute.", "Dogs are loyal.", "Birds are colorful."], "fish") == []
output: def search(documents, keyword):
      keyword_lower = keyword.lower()
      return [doc for doc in documents if keyword_lower in doc.lower()]