Update example Diagnostic and docs page

[lukruh]

The create a diagnostic part seems to be incomplete and the example shared/examples/diagnostic.py could be updated.
To avoid redundancy and keep thinks well structured I'd suggest to provide a minimal working example as code snippet in the documentation under "Making a recipe or Diagnostic" and refer to the tutorial where a more detailed step by step guide should be available and the example diagnostic as quickstart (similar with recipe).

The example diagnostic however could be updated to be more comprehensive and an example of good practice. i.e.:

• follow code style rules
• include module description and function doc strings for API documentation
• provide some documented config options
• pass **kwargs from recipe to plot functions
• import shared and preprocessor functions for demonstration
• write .nc files and figures
• create provenance