Improve `pages:` job options: Move publish to pages.publish

Problem

When enabling multiple pages jobs a new pages: job option was created, but the MVC version of this option is quite verbose and don't contains all pages-related ci options.

Proposal

• Move publish to pages.publish
• Use any value defined in pages.publish as an artifact automatically: Follow up issue #519495 (closed) created
• Update documentation to communicate the publish as deprecated in favour of pages.publish

Old:

my-pages-job: 
  script:
  - echo 'something'
  artifacts:
    paths:
      - some-folder
  pages:
    path_prefix: '/stg'
  publish: '/some-folder' 

New:

my-pages-job: 
  script:
  - echo 'something'
  pages:
    path_prefix: '/stg'
    publish: '/some-folder' 

Related to:

• !133624 (comment 1599881481)
• GitLab Pages Parallel Deployments (&10914 - closed)
• #398145 (closed)