docs: cut 6.1.0 versions for docs, admin_docs, developer_docs, components

Snapshots all four versioned Docusaurus sections at v6.1.0, cut from
master after the version-cutting tooling (#39837) and broken-internal-
links fixes (#40102) landed. Captures fresh auto-generated content and
freezes data dependencies so the historical snapshot stays correct.

Versioning behavior: lastVersion stays at current for every section,
so the canonical URLs (/docs/..., /admin-docs/..., /developer-docs/...,
/components/...) continue to render content from master. The current
version is consistently labeled "Next" with an unreleased banner, and
6.1.0 is a historical pin accessible only via its explicit version
segment.

Component playground: previously disabled: true in versions-config.json,
now enabled and versioned. The plugin block in docusaurus.config.ts
was already gated only by the disabled flag, so no other code changes
were needed to bring it back online.

Snapshot includes:
- All MDX content for the four sections.
- Auto-gen captured fresh: 74 database pages (engine spec metadata),
  ~1,800 API reference files (openapi.json), 59 component pages
  (Storybook stories).
- Data imports frozen at cut time into snapshot-local _versioned_data/
  dirs:
    versioned_docs/version-6.1.0/_versioned_data/src/data/databases.json
      (canonical 80-database diagnostics from master, preserved by the
      generator's input-hash cache)
    admin_docs_versioned_docs/version-6.1.0/_versioned_data/data/countries.json
    admin_docs_versioned_docs/version-6.1.0/_versioned_data/static/feature-flags.json
    developer_docs_versioned_docs/version-6.1.0/_versioned_data/static/data/components.json
- Import paths in deeply-nested files rewritten so they still resolve
  from one directory deeper inside the snapshot.

Verified via full yarn build: exit 0, no broken links surfaced by
onBrokenLinks: throw. Anchor warnings present are pre-existing on
master (community#superset-community-calendar) and unrelated.

docs/developer_docs_versioned_docs/version-6.1.0/extensions/overview.md

@@ -0,0 +1,59 @@
+---
+title: Overview
+sidebar_position: 1
+---
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Overview
+
+Apache Superset's extension system enables organizations to build custom features without modifying the core codebase. Inspired by the [VS Code extension model](https://code.visualstudio.com/api), this architecture addresses a long-standing challenge: teams previously had to fork Superset or make invasive modifications to add capabilities like query optimizers, custom panels, or specialized integrations—resulting in maintenance overhead and codebase fragmentation.
+
+The extension system introduces a modular, plugin-based architecture where both built-in features and external extensions use the same well-defined APIs. This "lean core" approach ensures that any capability available to Superset's internal features is equally accessible to community-developed extensions, fostering a vibrant ecosystem while reducing the maintenance burden on core contributors.
+
+## What are Superset Extensions?
+
+Superset extensions are self-contained `.supx` packages that extend the platform's capabilities through standardized contribution points. Each extension can include both frontend (React/TypeScript) and backend (Python) components, bundled together and loaded dynamically at runtime using Webpack Module Federation.
+
+## Extension Capabilities
+
+Extensions can provide:
+
+- **Custom UI Components**: New panels, views, and interactive elements
+- **Commands and Menus**: Custom actions accessible via menus and keyboard shortcuts
+- **REST API Endpoints**: Backend services under the `/extensions/` namespace
+- **MCP Tools and Prompts**: AI agent capabilities for enhanced user assistance
+
+## UI Components for Extensions
+
+Extension developers have access to pre-built UI components via `@apache-superset/core/components`. Browse all available components on the [UI Components](/developer-docs/components/) page and filter by **Extension Compatible** to see components available to extensions.
+
+## Next Steps
+
+- **[Quick Start](./quick-start.md)** - Build your first extension with a complete walkthrough
+- **[Architecture](./architecture.md)** - Design principles and system overview
+- **[Dependencies](./dependencies.md)** - Managing dependencies and understanding API stability
+- **[Contribution Types](./contribution-types.md)** - Available extension points
+- **[Development](./development.md)** - Project structure, APIs, and development workflow
+- **[Deployment](./deployment.md)** - Packaging and deploying extensions
+- **[MCP Integration](./mcp.md)** - Adding AI agent capabilities using extensions
+- **[Security](./security.md)** - Security considerations and best practices
+- **[Tasks](./tasks.md)** - Framework for creating and managing long running tasks
+- **[Community Extensions](./registry.md)** - Browse extensions shared by the community