Add workflow to comment on fork PRs

The comment should allow maintainers to trigger a preview deployment for
fork PRs more easily.

Author: st0012

.github/workflows/pr-preview-comment.yml

@@ -0,0 +1,37 @@
+name: Comment on Fork PRs
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  comment-on-fork-pr:
+    runs-on: ubuntu-latest
+    # Only run for fork PRs
+    if: github.event.pull_request.head.repo.fork == true
+    steps:
+      - name: Comment on PR with manual deployment instructions
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const prNumber = context.payload.pull_request.number;
+            const workflowUrl = `https://github.com/ruby/rdoc/actions/workflows/cloudflare-preview.yml`;
+            const branch = context.payload.pull_request.head.ref;
+
+            // Create a direct link that pre-fills the PR number input
+            const dispatchUrl = `${workflowUrl}/dispatch?ref=main&inputs%5Bpull_request_number%5D=${prNumber}`;
+
+            await github.rest.issues.createComment({
+              issue_number: prNumber,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: `## Cloudflare Preview Deployment
+
+                ⚠️ This PR is from a fork, so the preview deployment workflow doesn't run automatically for security reasons.
+
+                If you're a maintainer and want to preview this PR:
+
+                [➡️ Click here to run the workflow with PR #${prNumber} pre-filled](${dispatchUrl})
+
+                This will trigger a Cloudflare Pages preview deployment for this PR.`
+            });