chore: Introduce VPAT 2.4 WCAG template

What does this MR do?

• Move contents of accessibility/vpat.md to vpat/508.md
• Update accessibility/vpat.md content to link to both reports. In this way no existing link back will return a 404 or needs a redirect.
• Add vpat/wcag.md with VPAT 2.4 WCAG template content for 2.0 and 2.1
• Update accessibility/a11y.md links to VPAT 2.4 508 and WCAG pages
• Update nav.json to link to both VPAT 2.4 508 and WCAG pages

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

Closes #1325 (closed)