Sys - Improve docs

[adrian-moisa]

• Update & improve readme example. It skips over the document part. Loading the document should be first (Retrieving a Document). Include the stateful parent widget in the example.
• Update migration document with latest architecture changes.
• Summary of most important changes in the top of this changelog. Explain: We prefer an architecture that is easy to maintain.
• Document the rules
• Improve Docs. Architecture overview:
• JSON - delta - documentM - attributes - styles - compose - rules - refresh - build - docs and lines - editable line - test spans - renderbox - selection/markers/rectangles - inputs - callbacks
• Documents - Document the models graph and linked list
• Improve & Review all docs
• Add explanation of modules

[dx314]

Currently the docs are out of date, as EditorController.basic no longer exists. As someone new to the project I actually have no idea how to get started.

[adrian-moisa]

Yes, we need to patch that and a bunch of other places. It's leftover work from the previous large refactor

[adrian-moisa]

Finally we have a grand document to overview the entire editor architecture: Architecture Overview. Wished Quill had this document. It would have spared us half an year of work to fork and refactor Quill.