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/versioned_docs/version-6.0.0/intro.md

@@ -165,7 +165,7 @@ Try out Superset's [quickstart](https://superset.apache.org/docs/quickstart/) gu
 ## Contributor Guide
 
 Interested in contributing? Check out our
-[Developer Portal](https://superset.apache.org/developer_portal/)
+[Developer Docs](https://superset.apache.org/developer-docs/)
 to find resources around contributing along with a detailed guide on
 how to set up a development environment.