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/using-superset/creating-your-first-dashboard.mdx

@@ -31,7 +31,7 @@ your existing SQL-speaking database or data store.
 
 First things first, we need to add the connection credentials to your database to be able
 to query and visualize data from it. If you're using Superset locally via
-[Docker compose](/docs/installation/docker-compose), you can
+[Docker compose](/docs/6.0.0/installation/docker-compose), you can
 skip this step because a Postgres database, named **examples**, is included and
 pre-configured in Superset for you.
 
@@ -188,7 +188,7 @@ Access to dashboards is managed via owners (users that have edit permissions to
 Non-owner users access can be managed in two different ways. The dashboard needs to be published to be visible to other users.
 
 1. Dataset permissions - if you add to the relevant role permissions to datasets it automatically grants implicit access to all dashboards that uses those permitted datasets.
-2. Dashboard roles - if you enable [**DASHBOARD_RBAC** feature flag](/docs/configuration/configuring-superset#feature-flags) then you will be able to manage which roles can access the dashboard
+2. Dashboard roles - if you enable [**DASHBOARD_RBAC** feature flag](/docs/6.0.0/configuration/configuring-superset#feature-flags) then you will be able to manage which roles can access the dashboard
    - Granting a role access to a dashboard will bypass dataset level checks. Having dashboard access implicitly grants read access to all the featured charts in the dashboard, and thereby also all the associated datasets.
    - If no roles are specified for a dashboard, regular **Dataset permissions** will apply.