Use multiple Pages deployments for review apps

What does this MR do and why?

Now that Pages multiple deployments is moving to beta: gitlab-org/gitlab!165472 (merged), we can use it for review apps in this project.

This helps us get familiar with the feature and makes reviewing changes to the project easier.

For this MR:

• I've enabled multiple deployments for this project: https://docs.gitlab.com/ee/user/project/pages/#enable-multiple-deployments
• Tried to create a "review environment" per: https://docs.gitlab.com/ee/user/project/pages/#use-multiple-deployments-to-create-pages-environments, so that we get a link from the UI to the review app.

Note that these review apps should "auto clean": https://docs.gitlab.com/ee/user/project/pages/#auto-clean.

Screenshots, screen recordings, or links to review app

• Review app: https://new.docs.gitlab.com/review-mr-198/
• Environments: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/environments

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.

Merge request 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 merge request.