add workflow for deleting branches (#265)

Author: Rich-Harris

.github/workflows/docs-preview-delete-request.yml

@@ -0,0 +1,27 @@
+# https://github.com/sveltejs/svelte.dev/blob/main/apps/svelte.dev/scripts/sync-docs/README.md
+name: Docs preview delete request
+
+on:
+  pull_request:
+    branches:
+      - main
+    types: [closed]
+
+jobs:
+  dispatch:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Repository Dispatch
+        uses: peter-evans/repository-dispatch@v3
+        with:
+          token: ${{ secrets.SYNC_REQUEST_TOKEN }}
+          repository: sveltejs/svelte.dev
+          event-type: docs-preview-delete
+          client-payload: |-
+            {
+              "package": "cli",
+              "repo": "${{ github.repository }}",
+              "owner": "${{ github.event.pull_request.head.repo.owner.login }}",
+              "branch": "${{ github.event.pull_request.head.ref }}",
+              "pr": ${{ github.event.pull_request.number }}
+            }