How attach a Custom Field do it by code to a Custom Type Page I create by YAML files a Field Custom, using the next documentation: 

https://www.drupal.org/docs/8/api/entity-api/programming-custom-fields-into-your-content-type

In this tutorial attach the field to Custom Type "car_brand" created by code, in my case I need attach the custom field to the Custom Type "page", he come default in drupal.

How I can attach the field to "page" content type?

Thanks a lot.
You just have to create or change the file attaching the field to instead be attached to the page content type:

First we'll have to rename it: field.field.node.page.field_brand_information.yml

Secondly, we should be fine just changing all the references from car_brand to the page bundle instead:

# field.field.node.page.field_brand_information.yml
langcode: en
status: true
dependencies:
  config:
    - field.storage.node.field_brand_information
    - node.type.page
  module:
    - text
id: node.page.field_brand_information
field_name: field_brand_information
entity_type: node
bundle: page
label: 'Brand Information'
description: 'More specific information about the car brand'
required: false
translatable: false
default_value: {  }
default_value_callback: ''
settings:
  display_summary: false
field_type: text_with_summary


Generally this approach is quite tedious and I agree as also pointed out in the article that you rather do it through the UI and then export the files appropriately.