Add R documentation for sample selection models

[PhilippBach]

Description of the issue or change proposal

Add documentation and examples for sample selection models in R, for example

• https://docs.doubleml.org/stable/guide/models.html#sample-selection-models-ssm
• https://docs.doubleml.org/stable/examples/py_double_ml_ssm.html

Example Notebook for SSM in R

I added a draft for the notebook example in R: https://github.com/DoubleML/doubleml-docs/blob/p-docu-ssm/doc/examples/R_double_ml_ssm.ipynb

Comments, context or references

Deploy docu after releasing SSMs in R: DoubleML/doubleml-for-r#213