Add descriptions of possible output products in Documentation

[jkikstra]

The documentation at the moment still misses a clear description of all possible (CSV and XLSX) output files.

I propose to add a list with names of those files, what they contain, and when they appear as output.

This could be added, for instance in the following section:
https://climate-assessment.readthedocs.io/en/latest/general.html#expected-output