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

docs/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "spring-security-docs-antora",
+  "private": true,
+  "dependencies": {
+    "@antora/cli": "^3.1.0",
+    "@antora/site-generator-default": "^3.1.0"
+  }
+}