Combine doc versioning pages in nav

What does this MR do and why?

While looking for these docs today, I saw how they are all listed separately, while there is a Product availability details page that might work as a nice top-level page for the three related pages:

• https://docs.gitlab.com/ee/development/documentation/styleguide/availability_details.html
  • https://docs.gitlab.com/ee/development/documentation/versions.html
  • https://docs.gitlab.com/ee/development/documentation/feature_flags.html
  • https://docs.gitlab.com/ee/development/documentation/experiment_beta.html

Review app: https://combine-doc-versioning-pages-in-nav.docs.gitlab-review.app/ee/development/documentation/styleguide/availability_details.html

Screenshots, screen recordings, or links to review app

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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/gitlab-docs/-/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.