Bare HTML page renderer.

By "bare HTML page", we mean that the following hooks that allow for "normal" pages are not invoked:

• hook_page_attachments()
• hook_page_attachments_alter()
• hook_page_top()
• hook_page_bottom()

Examples of bare HTML pages are:

• install.php
• update.php
• authorize.php
• maintenance mode
• exception handlers

i.e. use this when rendering HTML pages in limited environments. Otherwise, use a @code _controller @endcode route, and return a render array. This will cause a main content renderer (\Drupal\Core\Render\MainContent\MainContentRendererInterface) to be used, and in case of a HTML request that will be \Drupal\Core\Render\MainContent\HtmlRenderer.

In fact, this is not only typically used in a limited environment, it even must be used in a limited environment: when using the bare HTML page renderer, use as little state/additional services as possible, because the same safeguards aren't present (precisely because this is intended to be used in a limited environment).

Currently, there are two types of bare pages available:

• Install (hook_preprocess_install_page(), install-page.html.twig).
• Maintenance (hook_preprocess_maintenance_page(), maintenance-page.html.twig).