Label PRs that only change an index.md file as invalid (#63776)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: subatoi

.github/workflows/check-for-spammy-prs.yml

@@ -50,6 +50,7 @@ jobs:
             const onlyRenames = files.length > 0 && files.every(
               f => f.status === 'renamed' && f.additions === 0 && f.deletions === 0
             )
+            const onlyIndexMd = files.length === 1 && files[0].filename.split('/').pop() === 'index.md'
 
             if (
               targetsNonMain ||
@@ -58,7 +59,8 @@ jobs:
               isEmptyCommit ||
               touchesTooMany ||
               isBlankLineEdit ||
-              onlyRenames
+              onlyRenames ||
+              onlyIndexMd
             ) {
               await github.rest.issues.addLabels({
                 owner,