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

mkdocs.yml

@@ -141,7 +141,11 @@ nav:
           - Locations: 'integrations/google-cloud-storage/locations.md'
       - LDAP:
           - Org Data: 'integrations/ldap/org.md'
-  - Plugins:
+  - OpenAPI:
+      - Schema-first plugins with OpenAPI (Experimental): 'openapi/01-getting-started.md'
+      - Generate a client from your OpenAPI spec: 'openapi/generate-client.md'
+      - Validate your OpenAPI spec against test data: 'openapi/test-case-validation.md'
+  - Plugins (Legacy):
       - Intro to plugins: 'plugins/index.md'
       - Create a Backstage Plugin: 'plugins/create-a-plugin.md'
       - Plugin Development: 'plugins/plugin-development.md'
@@ -151,10 +155,6 @@ nav:
       - Plugin Analytics: 'plugins/analytics.md'
       - Feature Flags: 'plugins/feature-flags.md'
       - Internationalization (i18n): 'plugins/internationalization.md'
-      - OpenAPI:
-          - Schema-first plugins with OpenAPI (Experimental): 'openapi/01-getting-started.md'
-          - Generate a client from your OpenAPI spec: 'openapi/generate-client.md'
-          - Validate your OpenAPI spec against test data: 'openapi/test-case-validation.md'
       - Backends and APIs:
           - Proxying: 'plugins/proxying.md'
           - Backend plugin: 'plugins/backend-plugin.md'
@@ -165,6 +165,7 @@ nav:
           - Publish private: 'plugins/publish-private.md'
           - Add to Directory: 'plugins/add-to-directory.md'
           - Observability: 'plugins/observability.md'
+      - New Backend System: 'plugins/new-backend-system.md'
   - Configuration:
       - Static Configuration in Backstage: 'conf/index.md'
       - Reading Backstage Configuration: 'conf/reading.md'