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

.github/workflows/deploy-docs.yml

@@ -0,0 +1,8 @@
+name: Deploy Docs
+on:
+  workflow_dispatch:
+jobs:
+  register:
+    if: false
+    steps:
+    - run: echo Workflow registered!