docs: redirect Headlamp install namespace from kube-system to headlamp

Updates install docs, READMEs, troubleshooting guides, and CI/CD tutorial
to reference Headlamp's own install namespace (headlamp) instead of
kube-system for where the Headlamp plugin/UI is installed.

Out-of-scope (left unchanged):
- Source code references to kube-system (controller workload location)
- Test files with kube-system in mock configs

Files changed:
- docs/getting-started/installation.md
- docs/getting-started/quick-start.md
- docs/troubleshooting/README.md
- docs/troubleshooting/common-errors.md
- docs/troubleshooting/controller-issues.md
- docs/troubleshooting/encryption-failures.md
- docs/troubleshooting/permission-errors.md
- docs/tutorials/ci-cd-integration.md
- docs/development/workflow.md

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/troubleshooting/permission-errors.md

@@ -202,11 +202,11 @@ Missing service access permission.
 
 ```bash
 # Check service access
-kubectl auth can-i get services -n kube-system
-kubectl auth can-i get services/sealed-secrets-controller -n kube-system
+kubectl auth can-i get services -n headlamp
+kubectl auth can-i get services/sealed-secrets-controller -n headlamp
 
 # Check proxy access
-kubectl auth can-i get services/proxy -n kube-system
+kubectl auth can-i get services/proxy -n headlamp
 ```
 
 #### Solution
@@ -563,7 +563,7 @@ TOKEN=$(kubectl create token sealed-secrets-ci -n ci-cd)
 
 # Use with kubeseal
 echo -n mysecret | kubeseal \
-  --controller-namespace=kube-system \
+  --controller-namespace=headlamp \
   --token="$TOKEN" \
   --format=yaml
 ```