Provide a docs example on how to create custom editor with multiple roots

[f1ames]

Extracted from https://github.com/ckeditor/ckeditor5-editor-decoupled/issues/21 (and blocked by #1445 ATM).

We have concluded (together with @pjasiun and @Reinmar) that the first step towards multiple root editor should be providing a sample in docs which will show:

• How to create a custom editor creator which enables usage of multiple roots.
• In the same time it should be a solid example showing how to create custom editor creator.

The sample itself should be added under Examples > Framework section:

And the tutorial with all the code in Framework > Guides > Architecture > Deep dive > User interface section: