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/configuration/networking-settings.mdx

@@ -51,7 +51,7 @@ Restart Superset for this configuration change to take effect.
 
 #### Making a Dashboard Public
 
-1. Add the `'DASHBOARD_RBAC': True` [Feature Flag](/docs/configuration/feature-flags) to `superset_config.py`
+1. Add the `'DASHBOARD_RBAC': True` [Feature Flag](/docs/6.0.0/configuration/feature-flags) to `superset_config.py`
 2. Add the `Public` role to your dashboard as described [here](https://superset.apache.org/docs/using-superset/creating-your-first-dashboard/#manage-access-to-dashboards)
 
 #### Embedding a Public Dashboard