ML: Preprocess data for training storybook recommender model

[jo-elimu]

• [ ] https://github.com/elimu-ai/ml-storybook-recommender/issues/1
• [ ] https://github.com/elimu-ai/ml-storybook-recommender/issues/10
• [ ] https://github.com/elimu-ai/ml-storybook-recommender/issues/11