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/sidebarTutorials.js

@@ -42,6 +42,7 @@ const sidebars = {
         'contributing/howtos',
         'contributing/release-process',
         'contributing/resources',
+        'contributing/pkg-resources-migration',
         'guidelines/design-guidelines',
         {
           type: 'category',
@@ -91,6 +92,7 @@ const sidebars = {
           collapsed: true,
           items: [
             'extensions/extension-points/sqllab',
+            'extensions/extension-points/editors',
           ],
         },
         'extensions/development',
@@ -129,7 +131,7 @@ const sidebars = {
     {
       type: 'link',
       label: 'API Reference',
-      href: '/docs/api',
+      href: '/user-docs/api',
     },
   ],
 };