[Docs] Add documentation for customizing the keyboard navigation system

[miroiu]

There's no documentation about customizing the keyboard navigation. We should document the following topics:

• adding, removing and activating existing keyboard navigation layers
• customizing the focus visual for editor elements (including connections)
• customizing the keyboard navigation gestures

The new system was implemented here: #224

[Weichenleeeee123]

Hi , I'm a newcomer here and i would like to take this task if there's no one had taken it.

[miroiu]

Hi, you can have it.