No CI if only for markdown changes

There is no point in running CI only for markdown changes.
Such as rust-itertools#802 (prepare releases) and rust-itertools#767 (Contributing.md)

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

Author: Philippe-Cholet

.github/workflows/ci.yml

@@ -2,7 +2,11 @@ name: CI
 
 on:
   pull_request:
+    paths-ignore:
+      - "**.md"
   merge_group:
+    paths-ignore:
+      - "**.md"
 
 jobs:
   check: