Deep dive for focus tracking

[Reinmar]

📝 Provide a description of requested docs changes

Some goals:

• Introducing what focus is and how important it is for an editor and UI/UX/a11y. Differentiating focus and selection.
• Introducing focus tracking in the engine:
  • Focus observer (mention what it is but also that it uses capturing and listens on the editable)
  • focus/blur events
  • various isFocused properties
• Introducing focus tracking in the UI:
  • Focus tracker
  • Focus cycler
• Introducing editor's focus tracker:
  • editor.ui.focusTracker
  • Why do we need it?
  • How do plugins use it (register there, listen there)?
• A real-life example of all that in practice. For instance, what happens step by step when you click in the editor, press Cmd+L, type the URL and submit it. Where is the focus at each step, what's fired and what's the state of most important properties.
• PS. I feel that we should also provide here an answer for "how to focus the editor" as this guide may end up high in SERP for focus oriented queries. Potentially, this question can be answered at the very beginning of this guide or very end.