Add API documentation to page-title.html.twig

Problem/Motivation

API documentation must be included in radix/src/kits/radix_starterkit/templates/content/page-title.html.twig.

Proposed resolution

Replace the existing documentation with text from the API:

{#
/**
 * @file
 * Theme override for page titles.
 *
 * Available variables:
 * - title_attributes: HTML attributes for the page title element.
 * - title_prefix: Additional output populated by modules, intended to be
 *   displayed in front of the main title tag that appears in the template.
 * - title: The page title, for use in the actual content.
 * - title_suffix: Additional output populated by modules, intended to be
 *   displayed after the main title tag that appears in the template.
 */
#}