Had time to hack on this now, so didn't wait to hear if it was desirable. Close if you need to 👍

Closes #3804

• sidebar: use native <details> for collapsible groups #3804

Fixes #3517, closes #3802 as obviated

• accessibility: interactive controls should not be nested #3517
• interactive controls are not nested [#3517] #3802

Fixes #3805

• sidebar: unexpected toggling of groups with headings linked to the current page #3805

3804 is the primary target of this PR. Preserving the behaviors identified in 3517 and 3805 would have taken additional work, so I took the opportunity to address them for "free".

Before

Sidebar collapsible groups used custom markup and JS. Carets are focusable, an accessibility problem.

After

• 3804: Sidebar collapsible groups use native <details> elements. Custom markup and JS is reduced.
• 3517: dropped the separately-clickable carets. See PR interactive controls are not nested [#3517] #3802 for details on rationale.
• 3805: Clicking linked text in a collapsible group's heading doesn't toggle the group.

To test

1. With the released version, replace docs/.vitepress/config/en.ts's sidebarGuide with the following.

   function sidebarGuide(): DefaultTheme.SidebarItem[] {
     return [
       {
         text: 'Introduction',
         collapsed: true,
         items: [
           {
             text: 'Group heading linked to "What is VitePress?"',
             link: 'what-is-vitepress',
             collapsed: true,
             items: [
               {
                 text: 'Another group heading linked to "What is VitePress?"',
                 link: 'what-is-vitepress',
                 collapsed: true,
                 items: [
                   {
                     text: 'What is VitePress?',
                     link: 'what-is-vitepress',
                   },
                 ],
               },
             ],
           },
           { text: 'Getting Started', link: 'getting-started' },
           { text: 'Routing', link: 'routing' },
           { text: 'Deploy' }
         ]
       },
       { text: 'What is VitePress?', link: 'what-is-vitepress',},
       {
         text: 'Writing',
         collapsed: false,
         items: [
           { text: 'Markdown Extensions', link: 'markdown' },
           { text: 'Asset Handling', link: 'asset-handling' },
           { text: 'Frontmatter', link: 'frontmatter' },
           { text: 'Using Vue in Markdown', link: 'using-vue' },
           { text: 'Internationalization', link: 'i18n' }
         ]
       },
       {
         text: 'Customization',
         collapsed: false,
         items: [
           { text: 'Using a Custom Theme', link: 'custom-theme' },
           {
             text: 'Extending the Default Theme',
             link: 'extending-default-theme'
           },
           { text: 'Build-Time Data Loading', link: 'data-loading' },
           { text: 'SSR Compatibility', link: 'ssr-compat' },
           { text: 'Connecting to a CMS', link: 'cms' }
         ]
       },
       {
         text: 'Experimental',
         collapsed: false,
         items: [
           { text: 'MPA Mode', link: 'mpa-mode' },
           { text: 'Sitemap Generation', link: 'sitemap-generation' }
         ]
       },
       { text: 'Config & API Reference', base: '/reference/', link: 'site-config' }
     ]
   }

2. Run pnpm run docs

3. Open http://localhost:<your port>/guide/what-is-vitepress in a browser

4. Confirm that the "Introduction" group is closed. [3804 regression check]

5. Toggle "Introduction" and its children open by clicking on the carets [3517]. Confirm that all items linked to "What is VitePress?" have the active color. [3804 regression check]

6. Toggle unlinked group headings, eg. "Writing". Confirm that they toggle when either the text or caret is clicked. [3517]

7. Confirm that the non-group item linked to "What is VitePress?" has the indicator. [3804 regression check]

8. Toggle "Introduction" > "Group heading linked to “What is VitePress?”" closed.

9. Confirm that hovering over "Introduction" > "Deploy", which is not linked, does not give it the hover color. [3804 regression check]

10. Confirm that hovering of the text "Group heading linked to “What is VitePress?”" gives it the hover color. [3804]

11. Confirm that hovering with the cursor over linked sidebar items gives them the hover color. [3804 regression check]

12. Click "Introduction" > "Routing"

13. Under "Introduction", click the text of the first item, "Group heading linked to "“What is VitePress?”". Confirm that the site navigates and the group stays closed. [3805]