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/components/ui/buttongroup.mdx

@@ -0,0 +1,88 @@
+---
+title: ButtonGroup
+sidebar_label: ButtonGroup
+---
+
+<!--
+    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.
+-->
+
+import { StoryWithControls } from '../../../../src/components/StorybookWrapper';
+
+# ButtonGroup
+
+ButtonGroup is a container that groups multiple Button components together with consistent spacing and styling.
+
+## Live Example
+
+<StoryWithControls
+  component="ButtonGroup"
+  props={{
+  expand: false
+}}
+  controls={[
+  {
+    name: "expand",
+    label: "Expand",
+    type: "boolean",
+    description: "When true, buttons expand to fill available width."
+  },
+  {
+    name: "className",
+    label: "Class Name",
+    type: "text",
+    description: "CSS class name for custom styling."
+  }
+]}
+  sampleChildren={[{"component":"Button","props":{"buttonStyle":"tertiary","children":"Button 1"}},{"component":"Button","props":{"buttonStyle":"tertiary","children":"Button 2"}},{"component":"Button","props":{"buttonStyle":"tertiary","children":"Button 3"}}]}
+/>
+
+## Try It
+
+Edit the code below to experiment with the component:
+
+```tsx live
+function Demo() {
+  return (
+    <ButtonGroup>
+      <Button buttonStyle="tertiary">Button 1</Button>
+      <Button buttonStyle="tertiary">Button 2</Button>
+      <Button buttonStyle="tertiary">Button 3</Button>
+    </ButtonGroup>
+  );
+}
+```
+
+## Props
+
+| Prop | Type | Default | Description |
+|------|------|---------|-------------|
+| `expand` | `boolean` | `false` | When true, buttons expand to fill available width. |
+
+## Import
+
+```tsx
+import { ButtonGroup } from '@superset-ui/core/components';
+```
+
+---
+
+:::tip[Improve this page]
+This documentation is auto-generated from the component's Storybook story.
+Help improve it by [editing the story file](https://github.com/apache/superset/edit/master/superset-frontend/packages/superset-ui-core/src/components/ButtonGroup/ButtonGroup.stories.tsx).
+:::