repurpose 5.6.x branch to provide local docs build

* remove unused workflows, scripts, and configuration (now handled by docs-build branch)
* upgrade Antora to 3.1 (and Node.js to 16)
* tune playbook settings
* reconfigure docs build for local build only
* add patch to support using linked worktree as Antora content source
* remove Antora extensions not needed for local builds

.github/workflows/algolia-index.yml

@@ -1,20 +0,0 @@
-name: Update Algolia Index
-
-on:
-  schedule:
-    - cron: '0 10 * * *' # Once per day at 10am UTC
-  workflow_dispatch: # Manual trigger
-
-permissions:
-  contents: read
-
-jobs:
-  update:
-    name: Update Algolia Index
-    runs-on: ubuntu-latest
-    if: ${{ github.repository == 'spring-projects/spring-security' }}
-    steps:
-      - name: Checkout Source
-        uses: actions/checkout@v2
-      - name: Update Index
-        run: ${GITHUB_WORKSPACE}/.github/actions/algolia-docsearch-scraper.sh "${{ secrets.ALGOLIA_APPLICATION_ID }}" "${{ secrets.ALGOLIA_WRITE_API_KEY }}" "${GITHUB_WORKSPACE}/.github/actions/algolia-config.json"

.github/workflows/deploy-reference.yml

@@ -1,34 +0,0 @@
-name: Build & Deploy Reference
-
-on:
-  repository_dispatch:
-    types: request-build-reference
-  schedule:
-    - cron: '0 10 * * *' # Once per day at 10am UTC
-  workflow_dispatch: # Manual trigger
-
-permissions:
-  contents: read
-
-jobs:
-  deploy:
-    name: deploy
-    runs-on: ubuntu-latest
-    if: ${{ github.repository == 'spring-projects/spring-security' }}
-    steps:
-      - uses: actions/checkout@v2
-      - name: Set up gradle
-        uses: spring-io/spring-gradle-build-action@v1
-        with:
-          java-version: '11'
-          distribution: 'adopt'
-      - name: Cleanup Gradle Cache
-        # Remove some files from the Gradle cache, so they aren't cached by GitHub Actions.
-        # Restoring these files from a GitHub Actions cache might cause problems for future builds.
-        run: |
-          rm -f /home/runner/.gradle/caches/modules-2/modules-2.lock
-          rm -f /home/runner/.gradle/caches/modules-2/gc.properties
-      - name: Build with Gradle
-        run: ./gradlew :spring-security-docs:antora --stacktrace
-      - name: Deploy
-        run: ${GITHUB_WORKSPACE}/.github/actions/algolia-deploy.sh "${{ secrets.DOCS_USERNAME }}@${{ secrets.DOCS_HOST }}" "/opt/www/domains/spring.io/docs/htdocs/spring-security/reference/" "${{ secrets.DOCS_SSH_KEY }}" "${{ secrets.DOCS_SSH_HOST_KEY }}"