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/README.md

@@ -19,15 +19,16 @@ under the License.
 
 This is the public documentation site for Superset, built using
 [Docusaurus 3](https://docusaurus.io/). See the
-[Developer Portal](https://superset.apache.org/developer_portal/contributing/development-setup#documentation)
+[Developer Docs](https://superset.apache.org/developer-docs/contributing/development-setup#documentation)
 for documentation on contributing to documentation.
 
 ## Version Management
 
-The Superset documentation site uses Docusaurus versioning with three independent versioned sections:
+The Superset documentation site uses Docusaurus versioning with four independent sections:
 
-- **Main Documentation** (`/docs/`) - Core Superset documentation
-- **Developer Portal** (`/developer_portal/`) - Developer guides and tutorials  
+- **User Documentation** (`/user-docs/`) - End-user guides and tutorials
+- **Admin Documentation** (`/admin-docs/`) - Installation, configuration, and security
+- **Developer Docs** (`/developer-docs/`) - Developer guides, contributing, and extensions
 - **Component Playground** (`/components/`) - Interactive component examples (currently disabled)
 
 Each section maintains its own version history and can be versioned independently.