[Docs] Clean-up docs about editors API for plugins

[pjasiun]

We need to clean-up API Docs of the editor class and all related interfaces.

I think that EditorWithUi interface should extend Editor interface. I'm not sure if it is possible when Editor is a class, but it means that maybe Editor should also be defined as separated class and interface. Then, I hope, it will be possible to define that one interface extends another.

It should be also well described what is the difference between editing and UI parts of plugins and which interface should be used in each case. Also, plugins docs should be updated.

Related: https://github.com/ckeditor/ckeditor5-core/issues/115, https://github.com/ckeditor/ckeditor5-core/issues/113, #303, #327, #488.

[oskarwrobel]

After #2930 Intro docs in ckeditor5 has changed because StandardEditor class was removed. It would be nice to verify this doc and check if everything is correct and up to date.

[Reinmar]

I cleaned up API docs for these interface and the editor class. I think it's fine now.