docs: address copilot review feedback

Fix links in viewing-catalog.md and filter-catalog.md to point to
the new frontend system catalog-customization.md instead of the old
guide. Fix the Kubernetes entity content extension ID to use the
correct `entity-content:kubernetes/kubernetes` format. Wrap raw
extension definitions in `createFrontendModule` in search and
scaffolder docs. Correct TechDocs addon installation instructions
to explicitly install addon modules rather than claiming
auto-discovery.

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

docs/features/kubernetes/installation.md

@@ -24,7 +24,7 @@ The Kubernetes tab is shown by default for entities where Kubernetes data is ava
 ```yaml title="app-config.yaml"
 app:
   extensions:
-    - entity-content:kubernetes:
+    - entity-content:kubernetes/kubernetes:
         config:
           filter:
             metadata.annotations.backstage.io/kubernetes-id: