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/components/index.mdx

@@ -0,0 +1,93 @@
+---
+title: Extension Components
+sidebar_label: 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.
+-->
+
+# Extension Components
+
+These UI components are available to Superset extension developers through the `@apache-superset/core/components` package. They provide a consistent look and feel with the rest of Superset and are designed to be used in extension panels, views, and other UI elements.
+
+## Available Components
+
+- [Alert](./alert.mdx)
+
+## Usage
+
+All components are exported from the `@apache-superset/core/components` package:
+
+```tsx
+import { Alert } from '@apache-superset/core/components';
+
+export function MyExtensionPanel() {
+  return (
+    <Alert type="info">
+      Welcome to my extension!
+    </Alert>
+  );
+}
+```
+
+## Adding New Components
+
+Components in `@apache-superset/core/components` are automatically documented here. To add a new extension component:
+
+1. Add the component to `superset-frontend/packages/superset-core/src/ui/components/`
+2. Export it from `superset-frontend/packages/superset-core/src/ui/components/index.ts`
+3. Create a Storybook story with an `Interactive` export:
+
+```tsx
+export default {
+  title: 'Extension Components/MyComponent',
+  component: MyComponent,
+  parameters: {
+    docs: {
+      description: {
+        component: 'Description of the component...',
+      },
+    },
+  },
+};
+
+export const InteractiveMyComponent = (args) => <MyComponent {...args} />;
+
+InteractiveMyComponent.args = {
+  variant: 'primary',
+  disabled: false,
+};
+
+InteractiveMyComponent.argTypes = {
+  variant: {
+    control: { type: 'select' },
+    options: ['primary', 'secondary'],
+  },
+  disabled: {
+    control: { type: 'boolean' },
+  },
+};
+```
+
+4. Run `yarn start` in `docs/` - the page generates automatically!
+
+## Interactive Documentation
+
+For interactive examples with controls, run Storybook locally — see the [Storybook documentation](/developer-docs/testing/storybook).