Add short sections on export of ML models to user guide and brief examples to gallery

### 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](https://docs.doubleml.org/dev/guide/learners.html); see https://github.com/DoubleML/doubleml-for-py/pull/159 & https://github.com/DoubleML/doubleml-for-r/pull/169 ➕ https://github.com/DoubleML/doubleml-for-py/pull/91 & https://github.com/DoubleML/doubleml-for-r/pull/74
- [ ] clustered standard errors ➡️ [Inference section](https://docs.doubleml.org/dev/guide/se_confint.html) , see https://github.com/DoubleML/doubleml-for-py/pull/116 & https://github.com/DoubleML/doubleml-for-r/pull/119
- [ ] Estimation of RMSE / evaluation of predictive performance / comparison of learners

### Comments, context or references

_No response_