docs: fix broken links, missing sidebar entries, and restore lost content

- Replace all /developer_portal/ links with /developer-docs/ in index.md
- Fix non-existent extension-project-structure references
- Fix sidebar links using old /docs/ paths (/user-docs/ instead)
- Add missing extension-points/editors and pkg-resources-migration to sidebar
- Restore Kubernetes debugging section lost during howtos migration
- Restore pkg-resources-migration.md deleted during bifurcation
- Fix versioned docs quickstart.mdx missing version prefix
- Update stale developer_portal references in README, DOCS_CLAUDE, intro.md
- Fix broken creating-viz-plugins link in intro.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/sidebarAdminDocs.js

@@ -53,7 +53,7 @@ const sidebars = {
     {
       type: 'link',
       label: 'Database Drivers',
-      href: '/docs/databases',
+      href: '/user-docs/databases',
       description: 'See User Docs for database connection guides',
     },
     {