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/development-setup.md

@@ -1012,7 +1012,7 @@ When contributing new React components to Superset, please try to add a Story al
 
 The topic of authoring new plugins, whether you'd like to contribute
 it back or not has been well documented in the
-[the documentation](https://superset.apache.org/docs/contributing/creating-viz-plugins), and in [this blog post](https://preset.io/blog/building-custom-viz-plugins-in-superset-v2).
+[the documentation](/developer-docs/contributing/howtos#creating-visualization-plugins), and in [this blog post](https://preset.io/blog/building-custom-viz-plugins-in-superset-v2).
 
 To contribute a plugin to Superset, your plugin must meet the following criteria: