docs: cut 6.1.0 versions for user_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), broken-internal-
links fix (#40102), and user_docs rename (#40171) all landed. With
the rename in place, all four sections now produce parallel-named
files at the docs/ root (no more bare `versioned_docs/` outlier).

Versioning behavior: lastVersion stays at current for every section,
so the canonical URLs (/user-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.

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:
    user_docs_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.
- developer_docs/extensions/overview.md snapshot has the FIXED
  ./mcp.md form (from #40102), so the SPA-nav 404 isn't baked into
  the 6.1.0 version.

Verified via full yarn build: exit 0, no broken links surfaced by
onBrokenLinks: throw.

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