[Docs] Develop requirements for viewer documentation

[AlexSurtees]

Documentation for the viewer covers a few main areas:

1. Contributing guidelines. These will be primarily manually written but may make reference to the API documentation. May include references to architecture diagrams etc.
2. API documentation (e.g. for npm viewer package, consumption of viewer package API by app/URL parameters). It's likely that this can be generated automatically from types and structured in-code comments, and hosted in an interactive document.
3. Developer documentation, e.g. in-line documentation for functions, etc. Lowest priority, structured in-code comments are a good starting point.