Nav: move docs testing page to subdir, add child pages

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Restructure and split docs testing pages (gitlab!145054 - merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:
  • shorter index page
    • markdownlint page
    • vale page

Related links

• Depends on Restructure and split docs testing pages (gitlab!145054 - merged) where the page split is happening
• Related to Track splitting docs testing page apart (gitlab#442483 - closed) for tracking purposes