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/helm-release.yaml

@@ -0,0 +1,65 @@
+name: Release Helm Chart
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - 'helm/unifi-network-operator/**'
+      - '.github/workflows/helm-release.yaml'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pages: write
+  id-token: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Configure Git
+        run: |
+          git config user.name "$GITHUB_ACTOR"
+          git config user.email "$GITHUB_ACTOR@users.noreply.github.com"
+
+      - name: Install Helm
+        uses: azure/setup-helm@v4
+        with:
+          version: v3.14.0
+
+      - name: Run chart-releaser
+        uses: helm/chart-releaser-action@v1.6.0
+        with:
+          charts_dir: helm
+          config: .github/cr.yaml
+        env:
+          CR_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+          CR_SKIP_EXISTING: true
+
+  publish-pages:
+    needs: release
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout gh-pages
+        uses: actions/checkout@v4
+        with:
+          ref: gh-pages
+          path: gh-pages
+
+      - name: Setup Pages
+        uses: actions/configure-pages@v4
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: ./gh-pages
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4