Add GitLab Pages predefined variables

What does this MR do?

Adds GitLab Pages predefined variables:
CI_PAGES -> GitLab Pages host (ie, gitlab.io)
CI_PAGES_URL -> Full URL to the GitLab Page (ie, https://adrianmoisey.gitlab.io/blog/

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42966

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer