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/workflows/pr-validation.yaml

@@ -0,0 +1,90 @@
+name: PR Validation
+
+on:
+  pull_request:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  lint-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: 'go.mod'
+          cache: true
+
+      - name: Run go fmt
+        run: |
+          if [ -n "$(gofmt -s -l .)" ]; then
+            echo "Go code is not formatted:"
+            gofmt -s -d .
+            exit 1
+          fi
+
+      - name: Run go vet
+        run: go vet ./...
+
+      - name: Run tests
+        run: go test -v -race -coverprofile=coverage.out ./...
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v4
+        with:
+          file: ./coverage.out
+          fail_ci_if_error: false
+
+  helm-lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Helm
+        uses: azure/setup-helm@v4
+        with:
+          version: v3.14.0
+
+      - name: Lint Helm chart
+        run: |
+          helm lint helm/unifi-network-operator \
+            --set unifi.url="https://test.local" \
+            --set unifi.password="test"
+
+      - name: Template Helm chart
+        run: |
+          helm template test-release helm/unifi-network-operator \
+            --namespace test \
+            --set unifi.url="https://test.local" \
+            --set unifi.password="test" \
+            > /tmp/rendered.yaml
+
+      - name: Validate rendered manifests
+        run: |
+          # Check that the rendered output is valid YAML
+          kubectl --dry-run=client apply -f /tmp/rendered.yaml
+
+  docker-build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build Docker image (test only)
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/amd64
+          push: false
+          tags: test:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max