docs: bifurcate documentation into user, admin, and developer sections

Major restructuring of documentation to separate concerns:

**New Structure:**
- `/docs/` - User-facing docs (intro, quickstart, databases, using-superset, faq)
- `/admin-docs/` - Administrator docs (installation, configuration, security)
- `/developer-docs/` - Developer docs (contributing, extensions, guidelines, testing)

**Changes:**
- Move installation, configuration, and security docs to admin_docs/
- Move contributing, extensions, guidelines, and testing to developer_docs/
- Rename developer_portal to developer_docs (with underscore to hyphen in URL)
- Add sidebarAdminDocs.js for admin documentation navigation
- Update versions-config.json with new doc sections
- Update docusaurus.config.ts with new plugins and redirects
- Update internal links in versioned docs (6.0.0) to use new paths
- Keep user-facing content (databases, using-superset, faq) in main docs

This separation makes it clearer which documentation is relevant for:
- End users exploring and visualizing data
- Administrators deploying and configuring Superset
- Developers contributing to or extending Superset

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/versioned_docs/version-6.0.0/quickstart.mdx

@@ -15,7 +15,7 @@ Although we recommend using `Docker Compose` for a quick start in a sandbox-type
 environment and for other development-type use cases, **we
 do not recommend this setup for production**. For this purpose please
 refer to our
-[Installing on Kubernetes](/docs/installation/kubernetes/)
+[Installing on Kubernetes](/docs/6.0.0/installation/kubernetes/)
 page.
 :::
 
@@ -74,9 +74,9 @@ processes by running Docker Compose `stop` command. By doing so, you can avoid d
 From this point on, you can head on to:
 
 - [Create your first Dashboard](/docs/using-superset/creating-your-first-dashboard)
-- [Connect to a Database](/docs/configuration/databases)
-- [Using Docker Compose](/docs/installation/docker-compose)
-- [Configure Superset](/docs/configuration/configuring-superset/)
-- [Installing on Kubernetes](/docs/installation/kubernetes/)
+- [Connect to a Database](/docs/6.0.0/configuration/databases)
+- [Using Docker Compose](/docs/6.0.0/installation/docker-compose)
+- [Configure Superset](/docs/6.0.0/configuration/configuring-superset/)
+- [Installing on Kubernetes](/docs/6.0.0/installation/kubernetes/)
 
 Or just explore our [Documentation](https://superset.apache.org/docs/intro)!