drupal/core/modules/help_topics/help_topics.module

<?php

/**
 * @file
 * Displays help topics provided by modules and themes.
 */

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Url;

/**
 * Implements hook_help().
 */
function help_topics_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.help_topics':
      $help_home = Url::fromRoute('help.main')->toString();
      $module_handler = \Drupal::moduleHandler();
      $locale_help = ($module_handler->moduleExists('locale')) ? Url::fromRoute('help.page', ['name' => 'locale'])->toString() : '#';
      $search_help = ($module_handler->moduleExists('search')) ? Url::fromRoute('help.page', ['name' => 'search'])->toString() : '#';
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Help Topics module adds module- and theme-provided help topics to the module overviews from the core Help module. If the core Search module is enabled, these topics are also searchable. For more information, see the <a href=":online">online documentation for the Help Topics module</a>.', [':online' => 'https://www.drupal.org/modules/help_topics']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Viewing help topics') . '</dt>';
      $output .= '<dd>' . t('The top-level help topics are listed on the main <a href=":help_page">Help page</a>. Links to other topics, including non-top-level help topics, can be found under the "Related" heading when viewing a topic page.', [':help_page' => $help_home]) . '</dd>';
      $output .= '<dt>' . t('Providing help topics') . '</dt>';
      $output .= '<dd>' . t("Modules and themes can provide help topics as Twig-file-based plugins in a project sub-directory called <em>help_topics</em>; plugin meta-data is provided in YAML front matter within each Twig file. Plugin-based help topics provided by modules and themes will automatically be updated when a module or theme is updated. Use the plugins in <em>core/modules/help_topics/help_topics</em> as a guide when writing and formatting a help topic plugin for your theme or module.") . '</dd>';
      $output .= '<dt>' . t('Translating help topics') . '</dt>';
      $output .= '<dd>' . t('The title and body text of help topics provided by contributed modules and themes are translatable using the <a href=":locale_help">Interface Translation module</a>. Topics provided by custom modules and themes are also translatable if they have been viewed at least once in a non-English language, which triggers putting their translatable text into the translation database.', [':locale_help' => $locale_help]) . '</dd>';
      $output .= '<dt>' . t('Configuring help search') . '</dt>';
      $output .= '<dd>' . t('To search help, you will need to install the core Search module, configure a search page, and add a search block to the Help page or another administrative page. (A search page is provided automatically, and if you use the core Seven administrative theme, a help search block is shown on the main Help page.) Then users with search permissions, and permission to view help, will be able to search help. See the <a href=":search_help">Search module help page</a> for more information.', [':search_help' => $search_help]) . '</dd>';
      $output .= '</dl>';
      return ['#markup' => $output];

    case 'help.help_topic':
      $help_home = Url::fromRoute('help.main')->toString();
      return '<p>' . t('See the <a href=":help_page">Help page</a> for more topics.', [
        ':help_page' => $help_home,
      ]) . '</p>';
  }
}

/**
 * Implements hook_theme().
 */
function help_topics_theme() {
  return [
    'help_topic' => [
      'variables' => [
        'body' => [],
        'related' => [],
      ],
    ],
  ];
}

/**
 * Implements hook_modules_uninstalled().
 */
function help_topics_modules_uninstalled(array $modules) {
  // Early return if search is not installed or if we're uninstalling this
  // module.
  if (!\Drupal::hasService('plugin.manager.search') ||
    in_array('help_topics', $modules)) {
    return;
  }
  $search_plugin_manager = \Drupal::service('plugin.manager.search');
  if ($search_plugin_manager->hasDefinition('help_search')) {
    // Ensure that topics for extensions that have been uninstalled are removed.
    $help_search = $search_plugin_manager->createInstance('help_search');
    $help_search->updateTopicList();
  }
}

/**
 * Implements hook_themes_uninstalled().
 */
function help_topics_themes_uninstalled(array $themes) {
  // Use the same code as module uninstall to ensure that theme help is removed
  // when a theme is uninstalled.
  help_topics_modules_uninstalled([]);
}