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/testing/storybook.md

@@ -0,0 +1,114 @@
+---
+title: Storybook
+sidebar_position: 5
+---
+
+<!--
+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.
+-->
+
+# Storybook
+
+Superset uses [Storybook](https://storybook.js.org/) for developing and testing UI components in isolation. Storybook provides a sandbox to build components independently, outside of the main application.
+
+## Public Storybook
+
+A public Storybook with components from the `master` branch is available at:
+
+**[apache-superset.github.io/superset-ui](https://apache-superset.github.io/superset-ui/?path=/story/*)**
+
+## Running Locally
+
+### Main Superset Storybook
+
+To run the main Superset Storybook locally:
+
+```bash
+cd superset-frontend
+
+# Start Storybook (opens at http://localhost:6006)
+npm run storybook
+
+# Build static Storybook
+npm run build-storybook
+```
+
+### @superset-ui Package Storybook
+
+The `@superset-ui` packages have a separate Storybook for component library development:
+
+```bash
+cd superset-frontend
+
+# Install dependencies and bootstrap packages
+npm ci && npm run bootstrap
+
+# Start the @superset-ui Storybook (opens at http://localhost:9001)
+cd packages/superset-ui-demo
+npm run storybook
+```
+
+## Adding Stories
+
+### To an Existing Package
+
+If stories already exist for the package, extend the `examples` array in the package's story file:
+
+```
+storybook/stories/<package>/index.js
+```
+
+### To a New Package
+
+1. Add package dependencies:
+
+   ```bash
+   npm install <package>
+   ```
+
+2. Create a story folder matching the package name:
+
+   ```bash
+   mkdir storybook/stories/superset-ui-<package>/
+   ```
+
+3. Create an `index.js` file with the story configuration:
+
+   ```javascript
+   export default {
+     examples: [
+       {
+         storyPath: '@superset-ui/package',
+         storyName: 'My Story',
+         renderStory: () => <MyComponent />,
+       },
+     ],
+   };
+   ```
+
+   Use the `|` separator for nested stories:
+   ```javascript
+   storyPath: '@superset-ui/package|Category|Subcategory'
+   ```
+
+## Best Practices
+
+- **Isolate components**: Stories should render components in isolation, without application context
+- **Show variations**: Create stories for different states, sizes, and configurations
+- **Document props**: Use Storybook's controls to expose configurable props
+- **Test edge cases**: Include stories for loading states, error states, and empty states