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 #290
• [ ] building-envelope-data/database#43
• [ ] building-envelope-data/metabase#112
• [ ] #106
• [ ] #119
• [ ] #118
• [ ] #237