Add GitHub Actions CI/CD workflows and documentation

- Add Docker image build and push workflow (multi-arch: amd64, arm64)
- Add Helm chart release workflow with GitHub Pages publishing
- Add comprehensive release workflow for version tags
- Add PR validation workflow (tests, linting, validation)
- Update Chart.yaml and values.yaml with GitHub URLs
- Update image repository to use ghcr.io
- Add detailed CI/CD documentation and setup guides

Workflows provide:
- Automated Docker image builds to GitHub Container Registry
- Automated Helm chart releases to GitHub Pages
- Complete release automation with version tagging
- PR validation with tests and linting

Helm repository will be available at:
https://vegardengen.github.io/unifi-network-operator

Docker images available at:
ghcr.io/vegardengen/unifi-network-operator

.github/cr.yaml

@@ -0,0 +1,6 @@
+# Chart Releaser configuration
+owner: vegardengen
+git-repo: unifi-network-operator
+charts-repo-url: https://vegardengen.github.io/unifi-network-operator
+# Skip packaging if the chart version already exists
+skip-existing: true