Document new layout's external names

What does this MR do?

Migrate layout names to use in external communications from https://gitlab.com/groups/gitlab-org/plan-stage/-/wikis/Technical-Decisions-and-Constraints:-Paneled-design#names-for-panels-to-use-in-ui-text-and-docs.

When this is merged, that wiki section should remain, but its only contents should be a link to this new location.

Does this MR meet the acceptance criteria?

• 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.
• Maintainer review follows the Pajamas UX maintainer review checklist.

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