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/docker-build-push.yaml

@@ -0,0 +1,92 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches:
+      - main
+      - 'feature/**'
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - main
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: 'go.mod'
+          cache: true
+
+      - name: Run tests
+        run: |
+          go test -v ./... -coverprofile=coverage.out
+          go tool cover -func=coverage.out
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,prefix={{branch}}-
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            VERSION=${{ github.ref_name }}
+            COMMIT=${{ github.sha }}
+            BUILD_DATE=${{ github.event.head_commit.timestamp }}
+
+      - name: Upload coverage reports
+        if: github.event_name == 'pull_request'
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: coverage.out
+          retention-days: 7