drupal/core/modules/help/help.module

<?php

/**
 * @file
 * Manages displaying online help.
 */

use Drupal\Core\Block\BlockPluginInterface;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function help_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.main':
      $output = '<h2>' . t('Getting Started') . '</h2>';
      $output .= '<p>' . t('Follow these steps to set up and start using your website:') . '</p>';
      $output .= '<ol>';
      $output .= '<li>' . t('<strong>Configure your website</strong> Once logged in, visit the <a href=":admin">Administration page</a>, where you may <a href=":config">customize and configure</a> all aspects of your website.', [':admin' => \Drupal::url('system.admin'), ':config' => \Drupal::url('system.admin_config')]) . '</li>';
      $output .= '<li>' . t('<strong>Enable additional functionality</strong> Next, visit the <a href=":modules">Extend page</a> and enable modules that suit your specific needs. You can find additional modules at the <a href=":download_modules">Drupal.org modules page</a>.', [':modules' => \Drupal::url('system.modules_list'), ':download_modules' => 'https://www.drupal.org/project/modules']) . '</li>';
      $output .= '<li>' . t('<strong>Customize your website design</strong> To change the "look and feel" of your website, visit the <a href=":themes">Appearance page</a>. You may choose from one of the included themes or download additional themes from the <a href=":download_themes">Drupal.org themes page</a>.', [':themes' => \Drupal::url('system.themes_page'), ':download_themes' => 'https://www.drupal.org/project/themes']) . '</li>';
      // Display a link to the create content page if Node module is enabled.
      if (\Drupal::moduleHandler()->moduleExists('node')) {
        $output .= '<li>' . t('<strong>Start posting content</strong> Finally, you may <a href=":content">add new content</a> to your website.', [':content' => \Drupal::url('node.add_page')]) . '</li>';
      }
      $output .= '</ol>';
      $output .= '<p>' . t('For more information, refer to the help listed on this page or to the <a href=":docs">online documentation</a> and <a href=":support">support</a> pages at <a href=":drupal">drupal.org</a>.', [':docs' => 'https://www.drupal.org/documentation', ':support' => 'https://www.drupal.org/support', ':drupal' => 'https://www.drupal.org']) . '</p>';
      return ['#markup' => $output];

    case 'help.page.help':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Help module generates <a href=":help-page">Help reference pages</a> to guide you through the use and configuration of modules, and provides a Help block with page-level help. The reference pages are a starting point for <a href=":handbook">Drupal.org online documentation</a> pages that contain more extensive and up-to-date information, are annotated with user-contributed comments, and serve as the definitive reference point for all Drupal documentation. For more information, see the <a href=":help">online documentation for the Help module</a>.', [':help' => 'https://www.drupal.org/documentation/modules/help/', ':handbook' => 'https://www.drupal.org/documentation', ':help-page' => \Drupal::url('help.main')]) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Providing a help reference') . '</dt>';
      $output .= '<dd>' . t('The Help module displays explanations for using each module listed on the main <a href=":help">Help reference page</a>.', [':help' => \Drupal::url('help.main')]) . '</dd>';
      $output .= '<dt>' . t('Providing page-specific help') . '</dt>';
      $output .= '<dd>' . t('Page-specific help text provided by modules is displayed in the Help block. This block can be placed and configured on the <a href=":blocks">Block layout page</a>.', [':blocks' => (\Drupal::moduleHandler()->moduleExists('block')) ? \Drupal::url('block.admin_display') : '#']) . '</dd>';
      $output .= '</dl>';
      return ['#markup' => $output];
  }
}

/**
 * Implements hook_theme().
 */
function help_theme($existing, $type, $theme, $path) {
  return [
    'help_section' => [
      'variables' => [
        'title' => NULL,
        'description' => NULL,
        'links' => NULL,
        'empty' => NULL,
      ],
    ],
  ];
}

/**
 * Implements hook_preprocess_HOOK() for block templates.
 */
function help_preprocess_block(&$variables) {
  if ($variables['plugin_id'] == 'help_block') {
    $variables['attributes']['role'] = 'complementary';
  }
}

/**
 * Implements hook_block_view_BASE_BLOCK_ID_alter().
 */
function help_block_view_help_block_alter(array &$build, BlockPluginInterface $block) {
  // Assume that most users do not need or want to perform contextual actions on
  // the help block, so don't needlessly draw attention to it.
  unset($build['#contextual_links']);
}