docs: document dummy-credential convention; annotate examples

Add CONTRIBUTING.md explaining the linting setup and how to mark the
repo's intentional dummy credentials so gitleaks does not flag them:
inline `# gitleaks:allow` for lint-clean, comment-capable files, and a
.gitleaksignore baseline otherwise.

Annotate the dummy credentials in four already-lint-clean example files
inline, as the preferred convention going forward.

- DIUN/docker-compose.yaml
- Headscale/Tailscale-Client/docker-compose,yaml
- Homepage/Homepage/services.yaml
- Kubernetes/Traefik-PiHole/Helm/Traefik/Dashboard/secret-dashboard.yaml

Homepage/Homepage/services.yaml

@@ -17,7 +17,7 @@
           type: traefik
           url: https://traefik.jimsgarage.co.uk
           username: admin
-          password: gT8ni3iX6QkKreWfAdYKe4xqVsaMRUQ4GG7xn59Q
+          password: gT8ni3iX6QkKreWfAdYKe4xqVsaMRUQ4GG7xn59Q  # gitleaks:allow
 
     - PiHole:
         icon: pi-hole.png
@@ -28,7 +28,7 @@
         widget:
           type: pihole
           url: http://192.168.8.2
-          key: 73T8oBs9MFKLVAC3mAs2KQbWSsqA7oe2PN9r9H4TQWg2TXNAdq4ZPzvy8oEv
+          key: 73T8oBs9MFKLVAC3mAs2KQbWSsqA7oe2PN9r9H4TQWg2TXNAdq4ZPzvy8oEv  # gitleaks:allow
 
 - My Second Group:
     - My Second Service: