feat(Settings): explain areas of settings within product and logic of where to place new settings

What does this MR do?

More explanation about what the different areas of Settings are in the GitLab product and how to determine where to place a new Setting. This is based off of conversations I've had with designers. When designing new settings, I didn't understand the logic of where a setting should live, I hope this MR starts to document the logic.

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.

Links

Epic to document Settings vision in Pajamas