Add CI Pages template to display static OpenAPI specification files in SwaggerUI

What does this MR do?

This request contains a gitlab-ci.yml template for Pages that sets up a static SwaggerUI as the project page showing the latest static OpenAPI specification file contents from a user specified documentation folder.

Addig this template provides help in setting up the first suggested solution in issue #4308 ("Swagger can be served, for example, using new GitLab Pages")

Example

• Example repo demonstrating the usage of the template: https://gitlab.com/mdhtr/gitlab-pages-swagger-ui
• Example resulting Swagger UI page: https://mdhtr.gitlab.io/gitlab-pages-swagger-ui/

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team