feat(settings): Add new region page for Settings

What does this MR do?

This MR add a new page under regions that documents Settings guidelines.

Originally proposed on #709 (closed), as part of the Settings UX Learnings & Recommendations gitlab-org/gitlab-design#1363 (closed), we want to provide a SSOT for our teams within Pajamas.

The goal is to empower guidance and reuse: designers want the out of the box ability to design with settings by reusing UI elements, layouts, as well as making use of documentation and settings best practices. This new page is a starting point that can be moved forward together with gitlab-org&4410

Does this MR meet the acceptance criteria?

• The MR title and commit message meet the Pajamas commit conventions
• Add a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Include MR scope that reflects the purpose of the change in the description
  • For example, if this MR is focused on usage guidelines, addressing accessibility challenges could be added separately with a different MR
• If creating a new page on Pajamas from scratch, follow the page template structure
• Follow the Pajamas Voice & Tone Guidelines and GitLab Documentation Style Guide
• Link components to and from other related components and patterns on the .md file, where helpful
• Assign it to a reviewer. All GitLab designers are reviewers, but you can also assign it directly to a maintainer or trainee maintainer
• Apply the correct component label(s)
• Select a milestone or due date

Links

Closes #709 (closed)