Add PagesDomains to OpenAPI docs

What does this MR do and why?

A tiny MVC for https://gitlab.com/gitlab-org/gitlab/-/issues/376954+

Screenshots or screen recordings

N/A

How to set up and validate locally

Documentation source: https://docs.gitlab.com/ee/api/pages.html

1. Run rake gitlab:openapi:generate
2. Review changes in doc/api/openapi/openapi_v2.yaml. You can check the content of the file by pasting it into https://editor.swagger.io. Errors will be shown on the left side.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #376954