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/developer_docs/contributing/resources.md

@@ -110,7 +110,7 @@ flowchart TD
 
 ## Entity-Relationship Diagram
 
-For the full interactive Entity-Relationship Diagram, please visit the [main documentation](https://superset.apache.org/docs/contributing/resources).
+For the full interactive Entity-Relationship Diagram, please visit the [developer documentation](/developer-docs/contributing/resources).
 
 You can also [download the .svg](https://github.com/apache/superset/tree/master/docs/static/img/erd.svg) directly from GitHub.
 
@@ -119,7 +119,7 @@ You can also [download the .svg](https://github.com/apache/superset/tree/master/
 ### Official Documentation
 - [Apache Superset Documentation](https://superset.apache.org/docs/intro)
 - [API Documentation](https://superset.apache.org/docs/api)
-- [Configuration Guide](https://superset.apache.org/docs/installation/configuring-superset)
+- [Configuration Guide](https://superset.apache.org/admin-docs/configuration/configuring-superset)
 
 ### Community Resources
 - [Apache Superset Blog](https://preset.io/blog/)