Docs: Document the model structure of all features

[oleq]

A follow–up of https://stackoverflow.com/q/54498805/1485219 (but also earlier questions on SO and Gitter).

I noticed that developers struggle to understand the structure of the content introduced by various features in the model. Understanding it requires browsing multiple *editing.js files and putting all things together, which takes time and does not feel straightforward.

Maybe we should come up with some:

• centralized comprehensive documentation of all features and their model structures (easy to find and read, but hard to maintain),
• standard of documenting the model structure introduced by each *Editing class — ATM such classes offer just a vague outline of what they introduce or they don't provide anything at all. For instance:

  /**
  * Foo plugin introduces the following model structure:
  *
  * <foo qux="String" [baz="Number"]>
  *  <bar>
  * </foo>
  *
  * @extends module:core/plugin~Plugin
  */
  export default class FooEditing extends Plugin {
     // ...
  }

  would do the trick. A Chrome developer tools extension for browsing the model would make things easier too but this is another story.

cc @Reinmar @AnnaTomanek @wwalc

[oleq]

Or maybe this kind of docs should land here?

[wwalc]

Kinda related: https://github.com/ckeditor/ckeditor5/issues/1205