Updated and extended version of user guide w.r.t. new features

We should update our user guide by adding brief demos and explanatations on

• export of ML models & predictions ➡️ Learners chapter; see Store estimated models for nuisance parameters doubleml-for-py#159 & Store estimated models for nuisance parameters doubleml-for-r#169 ➕ Provide an option to store & export the first-stage predictions doubleml-for-py#91 & Provide an option to store & export the first-stage predictions doubleml-for-r#74
• clustered standard errors ➡️ Inference section , see Cluster robust double machine learning doubleml-for-py#116 & Cluster robust double machine learning doubleml-for-r#119
• Estimation of RMSE / evaluation of predictive performance / comparison of learners

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