docs: migrate feature docs to new frontend system as primary content

Rewrite documentation for TechDocs, Software Templates, Software Catalog,
Search, and Kubernetes features to use the new frontend system as the
primary installation and configuration instructions. Old frontend system
instructions are moved to separate `--old` suffixed files for pages with
substantial legacy content, or updated inline for pages with minimal
old-system content.

Files migrated:
- techdocs/getting-started.md
- techdocs/how-to-guides.md
- software-templates/writing-custom-step-layouts.md
- software-templates/writing-custom-field-extensions.md
- software-templates/index.md
- software-catalog/catalog-customization.md
- search/getting-started.md
- search/how-to-guides.md
- kubernetes/installation.md

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

docs/features/kubernetes/installation.md

@@ -17,32 +17,20 @@ The first step is to add the Kubernetes frontend plugin to your Backstage applic
 yarn --cwd packages/app add @backstage/plugin-kubernetes
 ```
 
-Once the package has been installed, you need to import the plugin in your app by adding the "Kubernetes" tab to the respective catalog pages.
+Once installed, the plugin is automatically available in your app through the default feature discovery. It adds a "Kubernetes" tab to entity pages for entities that have Kubernetes resources associated with them. For more details and alternative installation methods, see [installing plugins](../../frontend-system/building-apps/05-installing-plugins.md).
 
-```tsx title="packages/app/src/components/catalog/EntityPage.tsx"
-/* highlight-add-next-line */
-import { EntityKubernetesContent } from '@backstage/plugin-kubernetes';
+The Kubernetes tab is shown by default for entities where Kubernetes data is available, based on the entity annotations. You can customize the entity filter for the tab through `app-config.yaml`:
 
-// You can add the tab to any number of pages, the service page is shown as an
-// example here
-const serviceEntityPage = (
-  <EntityLayout>
-    {/* other tabs... */}
-    {/* highlight-add-start */}
-    <EntityLayout.Route path="/kubernetes" title="Kubernetes">
-      <EntityKubernetesContent refreshIntervalMs={30000} />
-    </EntityLayout.Route>
-    {/* highlight-add-end */}
-  </EntityLayout>
-);
+```yaml title="app-config.yaml"
+app:
+  extensions:
+    - entity-content:kubernetes:
+        config:
+          filter:
+            metadata.annotations.backstage.io/kubernetes-id:
+              $exists: true
 ```
 
-:::note Note
-
-The optional `refreshIntervalMs` property on the `EntityKubernetesContent` defines the interval in which the content automatically refreshes, if not set this will default to 10 seconds.
-
-:::
-
 That's it! But now, we need the Kubernetes Backend plugin for the frontend to work.
 
 ## Adding Kubernetes Backend plugin