Add preview links comment to PRs

.github/workflows/preview-build.yml

@@ -43,12 +43,17 @@ on:
         type: string
         default: 'false'
         required: false
+      disable-comments:
+        description: 'Disable comments'
+        type: boolean
+        default: false
+        required: false
 
 permissions:
   id-token: write
   deployments: write
   contents: read
-  pull-requests: read
+  pull-requests: write
 
 jobs:
   match:
@@ -231,6 +236,81 @@ jobs:
           aws cloudfront create-invalidation \
             --distribution-id EKT7LT5PM8RKS \
             --paths "${PATH_PREFIX}" "${PATH_PREFIX}/*"
+
+      - name: Comment on PR
+        continue-on-error: true
+        if: startsWith(github.event_name, 'pull_request') && inputs.disable-comments != 'true' && env.MATCH == 'true' && steps.deployment.outputs.result && steps.check-files.outputs.all_changed_files
+        uses: actions/github-script@v7
+        env:
+          ALL_CHANGED_FILES: ${{ steps.check-files.outputs.all_changed_files }}
+        with:
+          script: |
+            const title = '## 🔍 Preview links for changed docs'
+            const changedMdFiles = process.env.ALL_CHANGED_FILES
+              .split(/\s+/)
+              .filter(i => i.endsWith('.md'))
+              .filter(i => !i.includes('/_snippets/'));
+
+            if (changedMdFiles.length === 0) {
+              return;
+            }
+
+            const toLink = (file) => {
+              const path = file
+                .replace('docs/', '')
+                .replace('/index.md', '')
+                .replace('.md', '');
+              return `[${file}](https://docs-v3-preview.elastic.dev${process.env.PATH_PREFIX}/${path})`;
+            }
+
+            const links = changedMdFiles.map(toLink)
+
+            const body = [
+              title,
+              ...links.slice(0, 10).map(i => `- ${i}`),
+            ]
+
+            if (links.length > 10) {
+              body.push('<details>');
+              body.push('<summary>More links …</summary>');
+              body.push('');
+              for (const link of links.slice(10, 100)) {
+                body.push(`- ${link}`);
+              }
+              body.push('');
+              body.push('</details>');
+            }
+
+            if (links.length > 100) {
+              body.push('');
+              body.push(`<sub>In total, ${links.length} files changed.</sub>`);
+            }
+
+            const owner = context.repo.owner;
+            const repo = context.repo.repo;
+            const issue_number = context.payload.pull_request.number;
+
+            // Post or update a single bot comment
+            const { data: comments } = await github.rest.issues.listComments({
+              owner, repo, issue_number
+            });
+            const existing = comments.find(c =>
+              c.user.type === 'Bot' &&
+              c.body.startsWith(title)
+            );
+            if (existing) {
+              await github.rest.issues.updateComment({
+                owner, repo,
+                comment_id: existing.id,
+                body: body.join('\n'),
+              });
+            } else {
+              await github.rest.issues.createComment({
+                owner, repo,
+                issue_number,
+                body:body.join('\n'),
+              });
+            }
 
       - name: Update Link Index
         if: |