Add link to data challenge 2 page.

docs/index.rst

@@ -12,6 +12,8 @@ In the following sections you will find:
 - Further usage examples
 - The Application Programming Interface (API), describes the various available classes, their properties, and usage.
 
+See also `COSI's second data challenge <https://github.com/cositools/cosi-data-challenge-2>`_ for the scientific description of the simulated data used in the tutorials, as well as an explanation of the statistical tools used by cosipy.
+
 .. warning::
    While many features are already available, cosipy is still actively under development. COSI is scheduled to launch in 2027. In preparation, the cosipy team will be releasing alpha versions with approximately an annual cadence. Your feedback will be greatly appreciated! Note, however, that these are not stable releases and various components can be modified or deprecated shortly.
 

docs/tutorials/index.rst

@@ -5,6 +5,8 @@ This is a series of tutorials explaining step by step the various components of
 
 If you are interested instead of the description of each class and method, please see our `API <../api/index.html>`_ section.
 
+See also `COSI's second data challenge <https://github.com/cositools/cosi-data-challenge-2>`_ for the scientific description of the simulated data used in the tutorials, as well as an explanation of the statistical tools used by cosipy.
+
 List of tutorials and contents, as a link to the corresponding Python notebook in the repository:
 
 1. Data format and handling `(ipynb) <https://github.com/cositools/cosipy/tree/main/docs/tutorials/DataIO/DataIO_example.ipynb>`_