input: Write a python function to search for a keyword in a list of documents.
  assert search_engine(["Hello world", "Python programming", "Search engine"], "Python") == ["Python programming"]
  assert search_engine(["Data science", "Machine learning", "Artificial intelligence"], "learning") == ["Machine learning"]
  assert search_engine(["Big data", "Data mining", "Data analysis"], "Data") == ["Big data", "Data mining", "Data analysis"]
output: def search_engine(documents, keyword):
      return [doc for doc in documents if keyword in doc]