input: Write a python function to perform sentiment analysis based on the presence of the words "great" and "bad" in a list of documents.
  assert sentiment_analysis(["This food was really great!"]) == ['positive']
  assert sentiment_analysis(["This movie was really bad."]) == ['negative']
  assert sentiment_analysis(["The day was great", "The weather was bad"]) == ['positive', 'negative']
  assert sentiment_analysis(["This is a neutral statement."]) == ['neutral']
  assert sentiment_analysis(["Great", "Bad", "Great", "Bad"]) == ['positive', 'negative', 'positive', 'negative']
output: def sentiment_analysis(documents):
      sentiments = []
      for doc in documents:
          if 'great' in doc.lower():
              sentiments.append('positive')
          elif 'bad' in doc.lower():
              sentiments.append('negative')
          else:
              sentiments.append('neutral')
      return sentiments