Improve the documentation of the Product Data Network

[christoph-maurer]

• more documentation at different levels of abstraction (graphical user interface, software architecture, software building blocks, code documentation); diagrams to aid in understanding the textual documentation; tests (from integration tests to unit tests); examples Develop a concept for the software development of all partners #290
• Document exemplary mutation queries database#43
• Create a tutorial for the endpoint metabase#112
• GraphQL HTML documentation #106
• JSON Schema documentation in Markdown or HTML #119
• Add further example queries (and other code) to the documentation of approvals in the database GraphQL schema #118
• Use spell checker and American English #237