Update documentation

Problem/Motivation

A follow-up of #3546356: Update documentation.

Update screenshots

Some screenshots:

• are old and don't represent the current state
• have low resolution and look "dirty"
• don't scale, especially in "Available islands"

So let's update them. With the new Drupal Core admin theme to be ready for next release.

Installation

The installation page can be a bit scary for people willing to discover the module:

• it describes the full installation of a Drupal with ddev
• it tells some recommended modules are required (UI Patterns Library, UI Styles...)
• it tells about UI Suite Bootstrap, giving the feeling the module is related to Bootstrap
• it described how to create a Page Layout and an Entity Display
• "Troubleshooting" is still mentioning display_builder_dev_tools which is not targeting our users >> move to FAQ and tell about drush command.

So it gives the feeling of a complicated install process.

Under the hood

• Move to specific section inside "Contibuting" so we keep the "Using Display Builder" pages only for user documentation
• Make sure Internals overview graph are all with white background. Transparent backgrounds don't look good on dark mode

Add a word about permanent storage in the FAQ.

Entity View Override

Update according to changes from #3562686: Entity view override enhancements.

Configuring

Maybe a little rewording about Island types (View, Menu, Contextual...).

Instances overview

There is nothing about this page added late in beta1, let's add something.