CI: Replace workflow with dispatch event to other repo

Saves the workflow file from being duplicated across two repositories.

.github/workflows/dispatch.yml

@@ -0,0 +1,27 @@
+name: Publish to Web
+
+on:
+  push:
+    branches:
+      - master
+  workflow_dispatch:
+
+jobs:
+  dispatch:
+    name: Trigger Deployment
+    permissions:
+      actions: write
+    runs-on: ubuntu-latest
+    steps:
+    - name: Dispatch guide.ubuntu-mate.org workflow
+      if: ${{ github.event_name == 'push' && github.repository == 'ubuntu-mate/ubuntu-mate-guide' }}
+      run: |
+        curl --fail -L \
+          -X POST \
+          -H "Accept: application/vnd.github+json" \
+          -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN }}" \
+          -H "X-GitHub-Api-Version: 2022-11-28" \
+          https://api.github.com/repos/ubuntu-mate/guide.ubuntu-mate.org/actions/workflows/main.yml/dispatches \
+          -d '{"ref":"master","inputs":{}}'
+        echo "View progress on the guide.ubuntu-mate.org repository:"
+        echo "https://github.com/ubuntu-mate/guide.ubuntu-mate.org/actions/workflows/main.yml"