Updated documentation as per latest missing features on about.gitlab.com

What does this MR do?

This merge request updates the documentation so it's corresponding with the current information on about.gitlab.com

CE-equivalent: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19134

Are there points in the code the reviewer needs to double check?

The confirmed wording for this documentation change is "GitLab is developed for Unix operating systems. It does not run on Windows, and we have no plans to support it in the near future. For the latest development status view this issue. Please consider using a virtual machine to run GitLab."

Why was this MR needed?

The old documentation was outdated and didn't provide any details about Windows support in the long term.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

No issues entered.