docs: sunset the docs/plugins section as legacy documentation

Mark all pages in docs/plugins/ with legacy admonition banners pointing
to the appropriate new frontend system and backend system documentation.
Rename the sidebar section to "Plugins (Legacy)" and move it lower in
the navigation hierarchy. Add cross-references from the new frontend
system building-plugins guide to relevant cross-cutting topics (i18n,
analytics, feature flags, search) that still live in the legacy section.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

microsite/sidebars.ts

@@ -412,56 +412,14 @@ export default {
     ),
     sidebarElementWithIndex(
       {
-        label: 'Plugins',
-        description: 'Extend Backstage with custom functionality.',
+        label: 'OpenAPI',
+        description:
+          'Work with OpenAPI specifications and generate clients.',
       },
       [
-        'plugins/index',
-        'plugins/create-a-plugin',
-        'plugins/plugin-development',
-        'plugins/structure-of-a-plugin',
-        'plugins/integrating-plugin-into-software-catalog',
-        'plugins/integrating-search-into-plugins',
-        'plugins/composability',
-        'plugins/internationalization',
-        'plugins/analytics',
-        'plugins/feature-flags',
-        sidebarElementWithIndex(
-          {
-            label: 'OpenAPI',
-            description:
-              'Work with OpenAPI specifications and generate clients.',
-          },
-          [
-            'openapi/01-getting-started',
-            'openapi/generate-client',
-            'openapi/test-case-validation',
-          ],
-        ),
-        sidebarElementWithIndex(
-          {
-            label: 'Backends and APIs',
-            description: 'Build and manage backend services and APIs.',
-          },
-          [
-            'plugins/proxying',
-            'plugins/backend-plugin',
-            'plugins/call-existing-api',
-          ],
-        ),
-        sidebarElementWithIndex(
-          { label: 'Testing', description: 'Testing plugins and modules.' },
-          ['plugins/testing'],
-        ),
-        sidebarElementWithIndex(
-          { label: 'Publishing', description: 'Publishing your plugins.' },
-          [
-            'plugins/publish-private',
-            'plugins/add-to-directory',
-            'plugins/plugin-directory-audit',
-          ],
-        ),
-        'plugins/observability',
+        'openapi/01-getting-started',
+        'openapi/generate-client',
+        'openapi/test-case-validation',
       ],
     ),
     sidebarElementWithIndex(
@@ -719,6 +677,50 @@ export default {
         ),
       ],
     ),
+    sidebarElementWithIndex(
+      {
+        label: 'Plugins (Legacy)',
+        description:
+          'Legacy plugin development documentation for the old frontend system. For new development, see the Frontend System and Backend System sections under Framework.',
+      },
+      [
+        'plugins/index',
+        'plugins/create-a-plugin',
+        'plugins/plugin-development',
+        'plugins/structure-of-a-plugin',
+        'plugins/integrating-plugin-into-software-catalog',
+        'plugins/integrating-search-into-plugins',
+        'plugins/composability',
+        'plugins/internationalization',
+        'plugins/analytics',
+        'plugins/feature-flags',
+        sidebarElementWithIndex(
+          {
+            label: 'Backends and APIs',
+            description: 'Build and manage backend services and APIs.',
+          },
+          [
+            'plugins/proxying',
+            'plugins/backend-plugin',
+            'plugins/call-existing-api',
+          ],
+        ),
+        sidebarElementWithIndex(
+          { label: 'Testing', description: 'Testing plugins and modules.' },
+          ['plugins/testing'],
+        ),
+        sidebarElementWithIndex(
+          { label: 'Publishing', description: 'Publishing your plugins.' },
+          [
+            'plugins/publish-private',
+            'plugins/add-to-directory',
+            'plugins/plugin-directory-audit',
+          ],
+        ),
+        'plugins/observability',
+        'plugins/new-backend-system',
+      ],
+    ),
     sidebarElementWithIndex(
       { label: 'FAQ', description: 'Frequently asked questions and answers.' },
       ['faq/index', 'faq/product', 'faq/technical'],