docs: replace hardcoded namespace with <your-namespace> placeholder

Users choose their own namespace for Headlamp. Replace all hardcoded
namespace references (headlamp, kube-system) in user-facing docs with
<your-namespace> so users substitute their own value.

Conventions:
- Helm install: --namespace <your-namespace> --create-namespace
- kubectl commands: -n <your-namespace>
- YAML metadata: namespace: <your-namespace>
- Prose: "the namespace where Headlamp is installed"

Out-of-scope references left untouched:
- kube-system in NetworkPolicy selectors (API server namespace)
- polaris namespace references (upstream workload namespace)
- Source code and test files

Refs: PRI-433

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

README.md

@@ -97,7 +97,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp # adjust to match your Headlamp service account
-    namespace: headlamp # adjust to match the namespace Headlamp runs in
+    namespace: <your-namespace>
 roleRef:
   kind: Role
   name: polaris-proxy-reader
@@ -197,7 +197,7 @@ npm test
 npm run test:watch
 
 # E2E tests (Playwright)
-export HEADLAMP_TOKEN=$(kubectl create token headlamp -n headlamp --duration=24h)
+export HEADLAMP_TOKEN=$(kubectl create token headlamp -n <your-namespace> --duration=24h)
 npm run e2e
 npm run e2e:headed   # see browser
 ```