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/contributing/development.mdx

@@ -599,7 +599,7 @@ export enum FeatureFlag {
 those specified under FEATURE_FLAGS in `superset_config.py`. For example, `DEFAULT_FEATURE_FLAGS = { 'FOO': True, 'BAR': False }` in `superset/config.py` and `FEATURE_FLAGS = { 'BAR': True, 'BAZ': True }` in `superset_config.py` will result
 in combined feature flags of `{ 'FOO': True, 'BAR': True, 'BAZ': True }`.
 
-The current status of the usability of each flag (stable vs testing, etc) can be found in the [Feature Flags](/docs/configuration/feature-flags) documentation.
+The current status of the usability of each flag (stable vs testing, etc) can be found in the [Feature Flags](/docs/6.0.0/configuration/feature-flags) documentation.
 
 ## Git Hooks
 
@@ -614,7 +614,7 @@ A series of checks will now run when you make a git commit.
 
 ## Linting
 
-See [how tos](/docs/contributing/howtos#linting)
+See [how tos](/docs/6.0.0/contributing/howtos#linting)
 
 ## GitHub Actions and `act`