Add search index workflow

- Relates #166

.github/workflows/rebuild-search-index.yml

@@ -0,0 +1,20 @@
+name: Rebuild Search Index
+on:
+  workflow_dispatch:
+permissions:
+  actions: write
+jobs:
+  build:
+    if: github.repository_owner == 'spring-projects'
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+      with:
+        ref: docs-build
+        fetch-depth: 1
+    - name: Dispatch
+      env:
+        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: gh workflow run rebuild-search-index.yml -r $(git rev-parse --abbrev-ref HEAD)
+