reconfigure branch for local builds and as scheduler for docs workflows

- set up placeholder and trigger for Deploy Docs workflow in docs-build branch
- set up placeholder and trigger for Rebuild Search Index workflow in docs-build branch
- remove obsolete Deploy Reference workflow
- upgrade Antora to 3.1
- reconfigure docs build for local build only
- add patch to support using linked worktree as Antora content source
- remove Antora extensions only needed for the production docs build

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"
+  }
+}