feat(DesignTokens): add placeholder design tokens page

What does this MR do?

Adds a design token page to allow additions from

• Design tokens > Create working definition (#1569 - closed)
• Design tokens > Define starting point and why (#1570 - closed)

Closes #1568 (closed)

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.

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