Add documentation

We want to have documentation for the plugin. I already started a general documentation located at docs/. (Please check out branch called docs, there you can find the added documentation files).

There are different topics in there:

• plugin setup
• input data
• output data

I already included the description of the plugin as well as the description of all parameters the plugin needs in docs/plugin_setup.md.

@b381851 Could you please add detailed information about the input data (climate indices produced by ClimDexCalc2 as well as more detailed information about model regions) in docs/input as well as more information about the output data in docs/output.

Please also add more, if you feel like there is something missing.