Add build infrastructure for Antora site.

See #810

.github/workflows/docs-build.yml

@@ -0,0 +1,61 @@
+name: Antora Site
+run-name: ${{ format('{0} ({1})', github.workflow, github.event.inputs.build-refname || 'all') }}
+on:
+  workflow_dispatch:
+    inputs:
+      build-refname:
+        description: Enter git refname to build (e.g., 5.7.x).
+        required: false
+permissions:
+  contents: write
+jobs:
+  build:
+    if: github.repository_owner == 'spring-projects'
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 5
+    - name: Set up JDK 17
+      uses: actions/setup-java@v3
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+    - name: Setup Maven Action
+      uses: s4u/setup-maven-action@v1.8.0
+      with:
+        java-version: 17
+    - name: Set up refname build
+      if: github.event.inputs.build-refname
+      run: |
+        git fetch --depth 1 https://github.com/$GITHUB_REPOSITORY ${{ github.event.inputs.build-refname }}
+        export BUILD_REFNAME=${{ github.event.inputs.build-refname }}
+        echo "BUILD_REFNAME=$BUILD_REFNAME" >> $GITHUB_ENV
+        export BUILD_VERSION=$(git cat-file --textconv FETCH_HEAD:pom.xml | python3 -c "import xml.etree.ElementTree as xml; from sys import stdin; print(xml.parse(stdin).getroot().find('{http://maven.apache.org/POM/4.0.0}version').text)")
+        echo BUILD_VERSION=$BUILD_VERSION >> $GITHUB_ENV
+    - name: Cache nodejs
+      uses: actions/cache@v3
+      with:
+        path: |
+          node
+          node_modules
+          package.json
+          package-lock.json
+        key: ${{ runner.os }}-nodejs-${{ hashFiles('pom.xml') }}
+    - name: Run Antora
+      run: ./mvnw -B antora -Dantora.playbook=playbooks/${{ github.event.inputs.module }}.yml
+    - name: Publish Docs
+      uses: spring-io/spring-doc-actions/rsync-antora-reference@v0.0.11
+      with:
+        docs-username: ${{ secrets.DOCS_USERNAME }}
+        docs-host: ${{ secrets.DOCS_HOST }}
+        docs-ssh-key: ${{ secrets.DOCS_SSH_KEY }}
+        docs-ssh-host-key: ${{ secrets.DOCS_SSH_HOST_KEY }}
+        site-path: target/antora/site
+    - name: Bust Cloudflare Cache
+      uses: spring-io/spring-doc-actions/bust-cloudflare-antora-cache@v0.0.11
+      with:
+        context-root: spring-vault
+        cloudflare-zone-id: ${{ secrets.CLOUDFLARE_ZONE_ID }}
+        cloudflare-cache-token: ${{ secrets.CLOUDFLARE_CACHE_TOKEN }}