Sidebar group headers should link to landing pages

[miharp]

Problem

Sidebar group headers render as plain, non-clickable text. Upstream Puppet docs (and VitePress generally) make a section header link to the section's landing page — for example, "The Puppet language" links to a language overview page.

The cause is in _includes/nav-item.html: when a nav entry has items:, the template renders the header and never checks link:, so a link: key on a group entry is silently ignored. The openbolt nav (_data/nav/openbolt_5x.yml) already declares link: on several group headers (Bolt, Plans, Tasks, Examples) and those links are dropped today.

Proposed fix (verified locally)

In the group branch of _includes/nav-item.html, render the header text as an anchor when link: is present, resolving the URL the same way the leaf-item branch does (absolute links starting with / pass through; others get the collection nav_base prefix).

This works with the theme as-is:

• The collapse handler in the theme JS ignores clicks on anchors (event.target.closest('a')), so clicking the header navigates while the caret still toggles the section.
• The theme CSS resets anchors to color: inherit; text-decoration: inherit;, so the header keeps its bold styling.

Verified with a local jekyll build: the header renders as a styled link and the openbolt group links resolve to existing pages.

First uses

• Point "The Puppet language" group in _data/nav/openvox_8x.yml at lang_summary.html (our language overview page).
• The openbolt group headers start linking automatically once the template honors link:.