Hook Menu Deprecated hook_menu alter too

hook_menu is used to create a custom page from your module till drupal 7 version , Now In drupal 8 its changed to yaml based, Basically now you have to create controller, routes.yml and basic info file for your module

Steps

1. First you need to create

yourmodulename    folder inside drupal 8 modules folder, then create info file thatis 

yourmodulename.info.yml

name: 'Your Module name'
description: 'Decription'
package: 'If needed'
type: module
core: 8.x

then create yourmodulename.routing.yml

yourmodulename.anything:
 path: '/anything'
 defaults:
  _controller: '\Drupal\yourmodulename\Controller\CustomController::someMethodInController'
  _title: 'Your custom title'
 requirements:
  _permission: 'any permission you want'

now create controller file 

everything you doing here is inside your module

now create a file inside your module in a path src\Controller\CustomController.php 

<?php

/**
 * @file
 * Contains \Drupal\youmodulename\Controller\CartController.
 */

namespace Drupal\youmodulename\Controller;
use Drupal\Core\Controller\ControllerBase;

/**
 * Contains the custom controller.
 */
class CustomController extends ControllerBase
{
 
  public function someMethodInController() {
    return array(
          '#type' => 'markup',
          '#markup' => "simply to say hello",
          );
  }

}

Enable your module and load your page at anything, Thats it