feat(nav-v2): wrap all content in Docs section; add Release notes, Troubleshoot, Reference

Restructures navigation-v2.yml so all existing labels are children of
a single "Docs" section (full V2 tree in one sidebar). Adds Release
notes, Troubleshoot, and Reference as separate section tabs.

Secondary nav bar is now left-aligned, compact, and fully data-driven
from section metadata.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: theletterf